The ScientologyToday web site has a section where Scientologists talk about Scientology.
It can be found here:
If you wander over that way, you'll find what some of the best known Scientology celebrities say about their religion.
And with that, you will probably understand why they are so outspoken about it.
I found, for myself, when I realized what Scientology really is, and how much it helps, I was eager to tell everyone about it.
In fact, I STILL am!