Why should we consider honoring the Sabbath, observing the biblical feast days, and keeping the Torah [the Law]?

Look for the things that Yehova said are forever, never-ending, for all generations, or a perpetual covenant.
Neither Paul nor anyone else has the authority to override what Yehova said stands forever.

Malachi 3:6
“For I am Yehova, I do not change…”
Numbers 23:19
Yehova is not a man who lies,
or a son of man who changes his mind!

If that isn’t enough, get a red letter bible and read only the words of Yeshua Messiah. Take notes.
You’ll find that Messiah teaches exactly what the Father teaches, which is His Torah, the Law.

John 7:16
16 Yeshua answered, “My teaching is not from Me, but from Him who sent Me.”
John 12:49-50
 “For I did not speak on My own, but the Father Himself who sent Me has commanded Me what to say and speak. 50 And I know that His commandment is life everlasting. Therefore what I say, I say just as the Father has told Me.”

If that isn’t enough, because of what you’ve been taught about Paul’s writings, read 2Peter 3:14-17. We are warned that misunderstanding Paul’s writings [and the rest of the scriptures] will result in our destruction — if we follow the error of the lawless.
¹⁴ Therefore, loved ones, while you are looking for these things, make every effort to be found in shalom, spotless and blameless before Him. ¹⁵ Bear in mind that the patience of our Lord means salvation—just as our dearly loved brother Paul also wrote to you with the wisdom given to him. ¹⁶ He speaks about these matters in all of his letters. Some things in them are hard to understand, which the ignorant and unstable twist (as they also do with the rest of the Scriptures)—to their own destruction.
¹⁷ Since you already know all this, loved ones, be on your guard so that you are not led astray by the error of the lawless and lose your sure footing.

If that isn’t enough, read everything Paul wrote.
Take notes.
You’ll find that he fully upholds, obeys, and supports obeying the Torah instructions (the Law), even though some of his writings sound like they are speaking against keeping the law (they are not).

Paul always kept the biblical Sabbath
Acts 17:2
As was his custom, Paul went to the Jewish people; and for three Shabbatot, he debated the Scriptures with them.
Acts 18:4
⁴ And he was debating every Shabbat in the synagogue, trying to persuade both Jewish and Greek people.

Paul believed ALL of the Torah
Acts 24:14
“But this I confess to you, that according to the Way (which they call a sect), I worship the God of our fathers, believing everything written in the Torah and the Prophets.”

Paul Stated that we are to uphold the Torah laws
Romans 3:31
Do we then nullify the Torah through faithfulness? May it never be! On the contrary, we uphold the Torah.

Paul taught from the Torah
Acts 28:23
²³ They set a day to meet Paul and came to him at his quarters in large numbers. From morning until evening he was explaining everything to them, testifying about the kingdom of God and trying to persuade them about Yeshua from both the Torah of Moses and the Prophets.

Paul obeyed the Torah
Acts 21:24
²⁴ Take them, and purify yourself along with them and pay their expenses, so that they may shave their heads. That way, all will realize there is nothing to the things they have been told about you, but that you yourself walk in an orderly manner, keeping the Torah.
Romans 7:25
²⁵ Thanks be to God—it is through Messiah Yeshua our Lord! So then, with my mind I myself serve the Torah of God; but with my flesh, I serve the law of sin.

Paul delights in the commands of the Torah [just like David, a man after God’s own heart]
Romans 7:22
²² For I delight in the Torah of God with respect to the inner man…”

Paul told us to imitate him, which would include knowing, believing, obeying, teaching, and delighting in the Torah.
And this is in the context of Paul imitating Messiah, walking just as He walked
1Corinthians 4:16
¹⁶ I urge you therefore—be imitators of me. 
1Corinthians 11:1
Be imitators of me, just as I also am of Messiah.

If that isn’t enough, you’ll have to decide which Paul you wish to follow.
Pro-Torah Paul
or
Anti-Torah Paul

If that isn’t enough, you can look at what the scriptures say about who will be in the kingdom (and who will not be in the kingdom):
Matthew 7:21-23
21 “Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of My Father in heaven. 22 Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ 23 And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
In the kingdom – those who do the will of the Father (Torah)
Not in the kingdom – those who practice lawlessness

Revelation 22:15-15
14 Blessed are those who do His commandments, that they may have the right to the tree of life, and may enter through the gates into the city. 15 But outside are dogs and sorcerers and sexually immoral and murderers and idolaters, and whoever loves and practices a lie.
In the kingdom – those who keep the commandments
Not in the kingdom – various sinners and those who love and practice a lie

What lies are mentioned in Revelation 22?
The lies that Jeremiah warned us about
Jeremiah 16:19-20
19 O Lord, my strength and my fortress,
My refuge in the day of affliction,
The Gentiles shall come to You
From the ends of the earth and say,
“Surely our fathers have inherited lies,
Worthlessness and unprofitable things.”
20 Will a man make gods for himself,
Which are not gods?

Still not good enough?
How about passing the tests given in Deuteronomy 13 and 1John 2
Deuteronomy 13
for the Lord your God is testing you to know whether you love the Lord your God with all your heart and with all your soul. 4 You shall walk after the Lord your God and fear Him, and keep His commandments and obey His voice; you shall serve Him and hold fast to Him.
1John 2
3 Now by this we know that we know Him, if we keep His commandments. 4 He who says, “I know Him,” and does not keep His commandments, is a liar, and the truth is not in him. 5 But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. 6 He who says he abides in Him ought himself also to walk just as He walked.

Still not enough?
How about simply listening to Messiah and following Him?
John 10:27-28
27 My sheep hear My voice, and I know them, and they follow Me. 28 And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.

Do you hear His voice and follow the things He says?
John 14:15
“If you love Me, keep My commandments.”
John 14:21
He who has My commandments and keeps them, it is he who loves Me.”
John 15:10
“If you keep My commandments, you will abide in My love, just as I have kept My Father’s commandments and abide in His love.”

Will you hear, “I never knew you. Depart from Me you workers of lawlessness”?
Or will you hear, “Well done good and faithful servant”?