Natural selection is the most compelling explanation of where
motivations originally came from. Why do we enjoy sex? Why do we
enjoy fast food? Why do we like sweets? Why do we see beauty in
savanna-like landscapes? Each of these questions requires a
non-circular response. The best one I've seen is that natural
selection has carefully crafted these preferences in our species.
Such preferences promoted survival and reproduction. Why would a
further answer than this be necessary? I suggest people read Stephen
Pinker's "The Way the Mind Works" for an excellent discussion of
these issues in evolutionary psychology.
In the end, there is no meaning. Meaning is in the journey. Don't
expect a completed life project. Probably no one has ever died
satisfied.