I'm with you on being wary. The thing is, when you're in a romantic (or intimate - whatever you want to call it) relationship, you have a lot more on the line than with family members. But it comes down to when you're married, your spouse becomes your family, and then wariness has no place.
When I was really getting to know someone, I wanted to make sure I was positive about how I felt and was pretty dang confident about how he felt, before the "I love you," step.
But it's all a crapshoot. You have no idea when you fall in love how things will turn out. You just do the best you can and choose wisely. Because that choosing part makes all the difference.