They’re always looking for something more interesting.
Sometimes things come up at the last minute, and people have to break plans. The vast majority of human beings are understanding when this happens. It is no one’s fault that the computer system crashed, ate your project and now you have to bail on dinner with friends so that you can recreate the report that is due tomorrow. It has happened to everyone. Most people also understand, even if they do not like it, if you accidentally double book yourself. You completely forgot that your sister’s baby shower is the same night that you promised to go to the movies with your friend. No one likes it when that happens, but if it occurs once in a blue moon, the vast majority of people will let it slide. If your friend, however, is constantly breaking plans, they may not be your friend at all. Friends make time for each other. They do not bail at the last minute unless it is unavoidable. It is always possible that your friend simply has rotten luck if they are breaking plans, but if they are canceling plans with you so they can hang out with someone else? That means that they are not actually double booked or making mistakes. They are simply on the lookout for someone they find more interesting than you.