Johann was convinced to go to America by these things: 1) Mr. Penn seemed like a practical, reasonable man, just like him, 2) if he comes he will get 450 acres!, a fortune in land, 3) he too wanted religious freedom, and 4) the soldiers were going to knock down his door and drag him away if he stayed much longer. Y'know, I think I'd go too.
As for how he felt later, I can tell you he was pretty pleased. In the narrative, the tone of voice describing his life there said as much. As well, he seemed determined to help William Penn create Pennsylvania. Making the Holy Experiment work was very important to him and at the end of the narrative he and Anna were talking about how amazing it was that so many religions were coexisting there. That shows he was serious about Pennsylvania.