Sometimes when I am standing in a row of urinals, I have trouble starting up if someone stands in the urinal next to me. If I have started peeing already I have no trouble continuing, but if I haven't started yet, it really throws me off my game when someone stands right next to me, even if I have to go real bad.
I hate people standing behind me when I'm going for a piss and it will give me bashful bladder sometimes.
The best ways to get around this problem is to wait until you are absolutely dying for a piss and then nothing is gonna stop you. Additonally, you could try getting drunk but be careful you don't piss on your neighbours leg.
I think this syndrome is an evolved reflex to make us on our guard when we are in the vulnerable state of having a nice satisfying slash down by the river when there be beasties lurking in the water.