Kangaroos evolved from opposums. Deer are artiodactyls, or placental mammals that have a double-pulley ankle joint, and most of the living species have an even number of toes (either 2 or 4), so they are also called even-toed ungulates. In fact whales also evolved from a primitive artiodactyl, so the deer are in fact more closely related to whales than they are to kangaroos. If there are any resemblance between the face of deer and kangaroos, it is strictly coincidental, not the result of inheriting the same features from a common ancestor. In fact the thylacine resembles the wolf even more closely than any resemblance between the kangaroo and deer. The resemblance is so remarkable that even university zoology students cannot easily tell a thylacine skull from a wolf skull.

The thylacine, however, is more closely related to the kangaroo and koala than it is to the wolf. Scientists call any such resemblance that evolved independently "convergent evolution. The resemblance between the wolf and the thylacine therefore is a remarkable example of convergent evolution.

Cal has the "what" part pretty well down, but you wanted the "why" part - Presumably it's because both deer and Kangaroos feed in a similar way on similar sorts of food. Of course, that's only part of the answer - the sort of skull that kangaroos and deer have is also similar to horses and even to Stegosaurs. Horses graze like kangaroos, while deer browse on the ends of twigs and branches as much as they graze. We're not entirely sure what Stegosaurs ate, but presumably more like deer than horses, since grass had not yet been invented - browsing on the tender twigs of shrubs and low trees seems likely. Convergent evolution is essentially adapting towards a 'best solution' to a particular problem - see sharks and dolphins or bats and birds.

http://www.boneclones.com/BC-100.htm

http://carchariastaurus.tumblr.com/post/...

http://www.sciencekids.co.nz/pictures/di...

http://commons.wikimedia.org/wiki/File:H...