1. If the sperm is not a good match for the egg.
2. If the sperm reaches out to the egg but does not hear back.
3. If the egg is occupied with another sperm.

Though it doesn’t have to be male=sperm, female=egg. It is so ingrained in our culture (and likely our evolution?) that the man pursues the woman, makes the first move. I’m fine with this and will make the first move, but starting with that move it must be a back and forth (tit for tat as mentioned above) going forward. I picture a tennis match where I hit the ball of interest onto her side of the court. Will she respond? How will she respond? When will she respond? Very fun and intriguing. But I am not hitting another ball over to her. Got to wait until she hits the first one back over to me (if it returns at all). And by tennis match I mean a back and forth partnership, not a competition. A respectful connection which may lead to friendship and/or more. Or nothing at all. Overall, I try to keep expectations low and respect high.