Because you know that the relationship is not going to work out or is currently destructive in some way. That you're wasting each other's time and preventing one another from finding more suitable partners. That although painful, it would be mutually beneficial to break up now so you both can move on with your lives.
Whether or not he's correct depends on his reasoning I suppose. For example he might think the two of you are funamentally incompatible. Or there might be external factors like distance, or family disapproval?
And yes I have broken up with girls for similar reasons in the past.
Is it burnin'? Well, f-ck, now you're learnin'.