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 is about reality, the actual truth. The facts are what they are, whether I believe them or not. Catholicism, or any other religion, states what it thinks is true or false, whether or not I -- or anyone else -- believe them, or like them. So, it wouldn't make any sense (nor would it be 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.
The fact is, everything the Church teaches, is true. It just is. I mean, Jesus said--"I am the way, the truth, and the light. No one comes to the Father except through Me." What more do you need?!