I give my boyfriend head on a regular basis. But whenever I do he always finishes himself off.. Does this mean I'm bad at giving head? Why does my boyfriend need to finish himself off?

Also, sometimes I'll be sucking him off and he's really enjoying it like hell moan a little and whisper under his breath things like "fuck" or "fuck baby". But then all of a sudden he moves me off of him and just buckles up his pants and puts it away and he hasn't even came? Is he just not attracted to me? Why doesn't he want me to keep going?