To put it simply, the Holy Bible made me do it.
I was raised in the United Methodist Church; I sang in the choir, and I was even the youth representative on the board of directors of the local church.
But I read the Holy Bible, and I found much that accorded with what I knew of Catholic teaching, and much that did not accord with Protestant beliefs.
As to the latter, James 2:24 teaches point-blank that justification is not by faith alone; moreover, Romans 3:28 does not teach justification by faith alone, but justification by faith: that is, without faith, no justification. (The Law that St. Paul refers to, and its works, are the Mosaic Law, and specifically circumcision.)
Of course, I understand now that there are a great many subtleties in redemption/justification/salvation theology, which I did not understand back then; I still view the Protestant sola fide as a distortion of biblical teaching, though.
Also, I noticed that the Bible itself teaches that the Sacred Scriptures are not the only source of divinely-inspired truth: see, for example, 2 Thessalonians 2:15. On the whole, Protestants do not believe this.
So, in my teenage reckoning, neither sola scriptura nor sola fide held up to biblical scrutiny. And they hold up even less now.
On the other hand, there is that great and awesome passage, the sixth chapter of John. The Lord Jesus Himself quite clearly insists that we must eat His Flesh and drink His Blood. (Having been a voracious reader since childhood, I knew well that those words were not metaphorical.) Methodists don't believe that; most Protestants don't believe that. But, Catholics and Orthodox do believe it, just as all the earliest Christians did.
And He also, quite clearly, gave the Apostles the authority to forgive sins: John 20:23. Protestants don't believe that; but Catholics and Orthodox do believe it. (I am sometimes flabbergasted at the linguistic/semantic/pseudo-logical contortions that Protestants go through to deny the clear meaning of the Lord's own words in John 6 and John 20).
And then there is Simon the Fisherman: Matthew 16:18 makes it quite clear that Simon was especially chosen by the Lord Himself to be the Rock on which He would build His Church; Luke 22:32 shows that the Lord intended Simon to have a special and essential role among the apostles; and John 21:15 evidences that the Lord Himself gave Simon a unique role as the pastor of Christians.
Protestants don't believe any of that; even the Orthodox don't believe any of that; but, Catholics do. (I am sometimes flabbergasted at the linguistic/semantic/pseudo-logical contortions that Protestants and Orthodox go through to deny the clear meaning of the Lord's own words in Matthew 16).
Well, that about sums it up (though, as I said above, this is altogether too brief and, therefore, inadequate). My motivation was, at least in my own mind, entirely biblical: I knew Catholics, of course, but I had never discussed religion with any of them; and, so far as I could recollect, I had never been to a Mass except to a wedding once when I was a young boy.