Negative sides about positivity

Nowadays the so called "Be positive, think positive" theory has gained much currency among people. I consider myself as a supporter of it in a sense, as I firmly believe that if we think positive things can definitely go better, even though I sometimes fail and fall in my wirlwind of negative thoughts. However, I get quite confused if not even upset when people talk about the necessity of being surrounded by positive people only. What if pessimistic friends need your help, will you leave them on their own? I think we should really put some limits to these new considerations as well as we should bear in mind our role: the more you tend to wish only positive people around you, the more you tend to be focused on yourself only rather than on other people. As a result, the most self-centred you become, which in my opinion is extremely dangerous and has no benefits at all. One should think positively for his own health, for his beloved ones, but narrowing down the list of people around you, just according to their mood is really selfish if not even cruel. Denying other people the possibility to be better, refusing them some help (which is a friend's duty and role) is cruel.