Simple.
Anyone who OAG likes is a BLB.
SS tends to attract cheaters, since he is, obviously, a scumbag.
OAG dumps BLB for SS, since BLB is so unlucky he can't even keep ANY girlfriend, even OAG.
SS is now stuck with OAG.
Anyone stuck with OAG is BLB.
SS becomes the new BLB.
Old BLB is now free of OAG, thus breaking his bad luck.