If It didn't want them coming back...

Thats a lot of IT's lol. In the novel what IT's experience with the lucky seven caused it to experience emotions for the first time. IT experiences pain, fear and anger, something IIt never experienced. From It's pov it was shocked that these kids had hurt him but it wasn't the pain he hated it was the fear, and because emotions aren't physical concepts IT could not destroy the feeling. Initially it wasn't a fear of the Lucky seven it was the fear of not being indestrutable and that there was a higher being than itself. Out of revenge and the need to rid IT's pangs of fear it wanted the lucky seven to return to Derry to finish them off and rid the feeling of fear. However, with the return of the Luky Seven the fear returned full force and then another horrible inconceivable feeling occurred, that for the first time IT thought that having the Lucky seven back was a mistake which leads to more fear.

Good answers. I would add one little detail.

It preys on children because children have such vivid imaginations, and thus have very different, primal fear reactions. Adults fear losing their job, getting divorced, getting a terminal illness, etc. Kids fear the monster under the bed. When the Losers managed to almost kill It, It believed that it was because they had turned their imaginations against It...that their great weakness was turned into a strength, largely through the interference of some "other" that It believed to be "The Turtle".

Later, when they are older, It brings them back. It does so because It has discovered that the Turtle is now dead, so It thinks it has a clear advantage because:

A) they are no longer kids, and no longer have such powerful imaginations
B) the Turtle is dead (It knows this and the Losers find out later)

It also helps It's confidence a bit when Stanley kills himself, because they are no longer seven, but six...and seven was a powerful lucky number.

The issue is that when they come back, they may not be kids and may be a person short, but they're still seriously pissed off, and have the experience of dealing with (and almost beating) It before. So as It reveals itself to scare (and then kill) them off, It finds that they aren't as scared as It hoped, and are still united and have some sort of power It still doesn't understand. It even begins to suspect some "Other" of pulling the strings, thinking that maybe it wasn't the Turtle after all. So It starts trying to scare them away, and gets Henry to try to kill them because as It reasons, if they can't be scared to death by illusions, they can still be killed by a physical being.

Whores will have their trinkets.