Sorrow x 2 ??

'Sorrow shared is sorrow halfed; Happiness shared is happiness doubled'

Why should shared sorrow be halfed?? When u share your sorrow with someone else, the burden of it is split with that someone isnt it? Then if that's the case, shouldnt the sorrow be divided and therefore doubled??

Why do we really feel the need to share our sorrows with others when times are bad? Are we humans really that weak or selfish?