They do not need to intend to touch you: bumping into someone ""by accident"" works, as does, say, a cashier's hand brushing against yours while they're giving you change. For that matter, even *you* don't necessarily have to intend to touch them: while the power will not activate by accident, if an accidental contact lasts long enough for you to have the thought process "hey, this is an opportunity to steal some lifeforce, I'm going to do that", you are free to act on said thought.

(Also, ouch.)