Throughout their lives, people imitate the model of loving behavior (or whatever they mistook for it) from their childhood.
Some people think that love is about concentrating their whole existence on a person, meeting their every need, and then feeling unappreciated and used when the object of “love” doesn’t respond in kind. They very specifically choose partners who need to feel like a disappointment because that’s what they, in turn, perceive as love.
Other people learned that love is sacrifice, and they will engineer situations where people will have to accept a degree of discomfort to “prove” their lives.
There are also those who learned that love is aggression. They will be angry, critical, and nitpicky only with the people they love. With everybody else, they’ll be total pussycats.
It’s always really fun to trace one’s own definition of love and look at its origins.