Basically, I am a Catholic because I believe that Catholicism is true.
Just to be clear, what I don't mean by this is that Catholicism is true for me. This is because religion is about more than just me -- it's about the objective nature of reality. The facts of the universe are what they are whether I believe them or not. That's what I mean by objective. And Catholicism, like any religion, makes claims about the objective nature of reality that are either true or false regardless of whether I -- or anyone else -- believe them, agree with them, or like them. Thus, it wouldn't make any sense (nor would it be intellectually honest) for me to say that Catholicism is true for me but not necessarily true for someone else. It's either true or it isn't.
And like I said, I believe that Catholicism is true, and for good reasons, I think. I'll have to go into what some of those reasons are a little later.