Why do people always want opposite of what they already have?

I always hear people bitch and complain about their partners’ behind their backs ,, and some of them will cheat or eventually or leave their partner for someone else , instead of fixing what is wrong in the relationship, thinking the other person is going to bring them happiness and love , not realizing the only reason the other person is better , is because they don’t live with them ,
Why do people always want opposite of what they already have?
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