Why is it so hard to make everyone happy without losing yourself in the process?

Is a jealous person jealous because, deep down they consider themselves the owner of the human being who is the object of their jealousy, or do they have some serious mental, cultural, moral, or intellectual problem, or a combination of all four?
Is it common to keep mementos, like photos, of a past love, and why do they hold such emotional significance over time?