Why do people ignore??

I hate people who ignore you. Especially after they were taking/ been replying quickly. Like I was talking to someone and he just talked about himself the entire time but the second I said something about me he just ignored it. And I know he's not asleep I know what he's doing. Like WTF if you don't want to talk then tell me??? God those people are so annoying and it's frustrating cause I can't not care I always overthink so I always get upset cause IDK maybe if feel like there's something wrong with me or something. UGGHH WHY DO PEOPLE DO THAT!!