I love and fully support the idea of code reviews, but I personally find it nearly impossible to review at anything less than the third level. As a result, code reviews take a lot of time and energy for me, and so I hypocritically tend to end up doing very few. It's just very hard for me to give my endorsement to a piece of code if I haven't had a chance to go over it completely.

Allocating more time for review would help, but I agree it's largely a matter of incentives. When push comes to shove, delivering the issues assigned to me is always going to take priority over reviewing someone else's code. I think that's only natural; issues resolved is generally the most visible metric for developer performance. I'm not sure how to change that in a meaningful way.