Tube security

On my way home tonight, there were police at my local tube station. Nothing unusual about that except they had an airport style metal detector and were stopping and searching young males.
I'm presuming they were looking for knives following the recent spate of teenage stabbings rather than terrorists. Should I feel any less worried?
Well so far it has been teenagers stabbing teenagers and I take a selfish comfort in the fact that my teen years are long gone. And presumably the extra police presence is always a good deterrent.
But why then does it still make me feel a bit nervous?