It depends on what you find out about the person in question, I'd say. I know that's the less compassionate answer, but practically speaking, it does depend, unless of course you're willing to invite just anybody into your life...which isn't terribly wise, I've found. ;) You could make generating certain emotions into a pretty technical process, like certain Buddhist meditations set out to do - generating loving kindness, compassion, joy and equanimity (the four immeasurables). Or, I think you could just jump in and try it out, try trusting someone whom you've met and had some experience with, and maybe things'll turn out alright.

Because people have a tendency to doubt, to be bored and always look for better options. For me the crucial question is whether I can trust myself in the first place) Anyway, you can never be sure about anything, and that's a double-edged sword)

Thanks @Andrés @Anastasia: yes you are right. I guess trust in someone is based always on so many factors. On the way the person is treating you or treated you, what past I have and which experiences I made as well as the person. Why is there no way to love without suffering? Why is such a warm feeling so close connected to such a dark one ?

It seems to me that it's better to take the risk and try, but you really have to experience quite strong feelings towards the person you're in love with. Being hurt is a fair price for negative result, because in case of positive one you'll get the prize that outweighs whatever doubts you have deep down your heart. Because ultimately love is everything we crave.