Why does he go to India of all places?

Because this are the places you look for when you are disillusioned. Other cultures and not many are left, especially so easy to travel ones like East or South-East Asia.

The movie was based on a book, so it loosely followed the same plot.

I say loosely because in the book the character who lived in the slums was a Polish Priest and he reasoned that being in that position would bring him closer to God. The American Doctor was more like the character Pauline Collins played.