Why do guys last a long time with someone they dont really like and then last a very short time and say they love you when they start getting serious with you?

Well because they're probably using the someone that they don't love and when they're with someone that they love then they find it harder to get along with that person because when they get into arguments,they find it hard to make amends with that someone that they love and that's when it get's intense.Hope that I helped.

Actually i think that what u're saying isn't true. Because if they were really serious with you then they wouldn't want to let you go and they would almost give up there life for you and only you. Nowadays guys are becoming more and more ego so its annoyiing so yeah...