Why are guys so obnoxious like this???

Ok, i was in my physics cl and we were doing a experiment outside of cl and this guy keep on walking behind me or when he talked to me he was looking at my chest. When i was in front of him walking back to cl and my friends told me he was staring at my but and makign comments about how its so big. I get this all the time and even more now since i gained more weight in my backside.