I think it's more of a choice then anything else for me.
I've been hurt in the past but yet I choose to trust people, knowing that I might get hurt again. I believe that loving and loosing is better then not loving at all.
I have a friend who's very untrusting, he suspect everyone, every time he leaves his car he check all the door 8 times, and would never leave his bag in a cafe while going to the bathroom, he check the change he get back in a store always and always think someone's trying to short change him.
I'm not like that, he never had his wallet stolen, I had mine stolen twice, on both case, nice people who found it, returned it to me. Even at the price of having my purse stolen a couple of time in my life, I rather trust humanity to return it or at least not have to stress out about it all the time.
I was upset about my stolen purse for a couple of weeks, he's upset about his not-yet-stolen wallet almost every day. I rather take the heartache of loosing something, then the constant worry and suspiciousness and isolation of mistrust.
I guess it's the same for larger things then purses and wallets.