Can I get some boy advice?

So I'm recently dating a very lovely guy and we are doing great together! However whenever I'm single only a few guys at a time seem to be interested in me but now that I'm not I've noticed that I have more and more guys come up to me and flirt and ask for my number. They don't know I'm in a relationship and I always make sure to tell them that so they don't waste their time, but I'm just wondering why is it when you're in a relationship more guys are attracted to you then when you are single?