Is it best to treat new acquaintances with compassion or skepticism?

I have an overactive sense of compassion (for lack of better terms) – and I have been made a sucker many times because of it.

Instinct tells me to be more reserved with it around unfamiliar people – even if they appear to be hurting. I’ve tried it, but I always feel guilty or get the impression I am behaving cruelly during and after.

Keep in mind, I’m not trying to brag here or even gain pity. Its a serious problem that has been bothering me for awhile. I would really appreciate some advice on which direction to go.