It's all about the fun for me, too.
I really didn't intend to build an AR, as I already had one with all the features I wanted--purchased complete.
Then I stumbled across a stripped lower at a price I couldn't refuse. It was all downhill from there.
When I realized I felt more attached to the AR I'd built from cobbled together--albeit excellent quality--parts than I did to my "perfect" pre-made rifle I knew I had the building bug.