A buddhist would say because we tend to cling to things being a certain way.

I think a better way to look at it is that life is full of challenge. You can either learn how to get comfortable dealing with certain challenges and stick to those, or dig into new territory.

If you're comfortable with the challenges you expose yourself to on a regular basis then it really wouldn't be difficult - sadly for me, I get bored easily and so far I've not found a way to stick to doing the same for very long.