how long have you had him? did you get him at a pet store?
if you just recently got him, he may still be settling in.
if you got him at a pet store, he may be older and not feel like moving around alot.
otherwise, he could just feel safe there or just like it there. chris's favorite place is laying on top of the leaves on his plant while shadow likes to sleep on his pillar.
i'm not sure what else would cause it.