Sometimes if a person has been in a relationship with someone for a while, sex, like anything one does many times, while still being good, one may become complacent about it. If the person you are in a relationship with has been irritating you, or you have had some spat with them, afterwards when you have sex with them, the feelings of complacency (which can at times make sex feel less exciting) can be altered.
Part of the definition of complacency is : often without awareness of some potential danger or defect; self-satisfied.
If you feel your relationship is at risk, even if it is only at some small level, it can make sex seem more exciting, as you appreciate or remember that you are pleased or grateful to be having sex.
It can also be cathartic to move from irritation to something of mutual satisfaction and pleasure.
If the fight has been a huge one, then no, make up sex while people are still very unhappy is not either possible or advisable. If you are able to sort things out at bit, then it may be become possible.
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Nothing lasts but nothing is lost