Because as humans with our faults sadly we take things for granted. No matter what it is. The sunrise as it clears the horizon. Or a valued relationship that we interact with every day. You dont know what it means to you till you dont have it anymore.. and when we are active in the moment our thoughts are selfserving..before they are considerate. So we then have regret going along with missing them.
I think we as humans, some of us, do not realize the value in love, friendship and talent. It’s like when a famous person passes away, they blow up. Everybody wants to hear their music or find out about them then. There’s an old saying, “You don’t missed the well, until it’s dry”! Some of us don’t appreciate the good in a person until they are no longer getting the good from that person anymore. I think I may have went off a bit on this question. Anyway, great question!