Why do guys leave you then regret it later?

Some guys feel as if they're not ready yet so they leave you and then when they're ready, they regret it. Or some guys found someone else but later on, you're still the one they love the most so they regret it. Some has peer pressures, some have family issues. Or some just are too blind too see. Some of them take advatage of girls and think their always going back to them when they're not and they regret that.