All the reasons above, but there a few other special reasons:
If a running joke starts in a thread and someone keeps it going, I'll dap.
If someone mentions being Haitian, I dap them to support my own.
A lot of times, I'll dap posters from Detroit or Michigan at least once a thread to support my own.
If someone makes a wrestling reference in a thread outside of TSC, I'll dap.
I dap good aliases.
@Hollywood Hogan is auto-dap about 90% of the time.
If I someone has a really interesting reflection and it's well-written with spacing/punctuation/grammar/vocabulary, the teacher in me daps it out of appreciation.