I personally do peer review because I am part of a research community that requires it -- 9 of every 10 articles I review are absolute crap, and the community is better off if only 3-5 reviewers waste time reading those articles. The only way to avoid a free rider problem is if everyone agrees to participate, so I participate. It is also a good way to build a career in research -- reliable peer reviewers will eventually be asked to do more visible things like chair conference sessions.
Would it be nice to be paid? Maybe, although to be honest I would rather keep money out of the process entirely -- I would like to continue having peer review be voluntary, and go further by also scrapping the publishing companies (who add nothing of value to any article I have written or seen in my entire career). In my field (cryptography) we run a preprint archive on a volunteer basis and it would not be a huge step to introduce a formal peer review process (there is already a minimal review process where the eprint vounteers reject papers that are obviously crap). We only bother with Springer because the European professors demand it (more precisely, their universities demand one of a handful of publishers, and Springer is least bad of the bunch).