I assume that when you were with them, something about them annoyed you. When they were gone, that annoyance wasn't there anymore, so you kinda forgot about it. And of course, then you missed the actual good things about them that you previously took for granted. It's hard to give advice without knowing exactly what you missed about them and exactly why you didn't really like them all that much when you were with them, but if you're trying to figure out whether to date someone, then I would write it all out on paper and look at the total package. Also, it really is okay not to want to date someone even if you kinda miss them when you're not together. Dating is entirely voluntary and it is perfectly fine if you're just not that into someone, even if they are a good person.