Why can some people get so cynical about ex-partners getting back together...?

Recently I got back together with my ex-boyfriend - we split up for about three weeks after three years of being together because I wanted some time apart from him for personal reasons (I realised that wasn't what I needed after all and we missed each other more than ever during our split). Thing is, now people know we're back together, many of them are a bit funny about it and basically aren't very supportive at all, even though I'm happier now than I have been for a long while. For instance, a few of my friends think I need to cut myself off from him completely and cut him out of my life - but I love him, and he feels the same! Why can some people get like this - I've noticed it with other people that've got back together as well - when really they should just be happy for you to be reunited with somebody you love...? I realise that breaking up was a mistake, I just had a lot to learn about myself. By the way, I'm 21 and he's 23.