I gave a “poor” reason : I just do.
I am convinced that belief in God is a grace of God. Of course, there are oodles of proof of the existence of God, but the fact is that, of two individuals with the same upbringing, intelligence and circumstances, one will believe and the other will not. There is therefore something else at work. It may be a function of free will, but, in this case, we know that we cannot even have a good thought without the grace of God.
I am not in the least troubled by unbelievers and if one asked me to prove the existence of God, I would simply answer : the great majority of people in the world do believe in a supreme being and if you don’t, the onus is on you to prove the opposite… **There was a time when most of the world believed that the earth was flat and that the sun went around it. They believed their senses, and they were right to do so. When people came around who thought differently, they were asked, rightly, to bring convincing proof. The same goes for the existence of God. **
We cannot convert anyone, only God converts. And if we are brought into contact with an unbeliever, God may or may not use us. Our job is to be a witness to our faith by our words and deeds. The kindness and understanding we show may go farther than any proof.