I'm afraid we spend too much time searching for this "soul mate" person, and as soon as someone comes along who matches a couple of the arbitrary guidelines we've set, we decide "That's the one" and fill in the blanks in our own mind.
Loneliness and hormones can create all kinds of interesting garbage in your brain, from the OMGwehavethisCONNECTIONthatIcan'texplain thing to some variation on "when he/she isn't drinking and cussing me out, he/she is really a good person".
Watch how they treat others -- this is how they will treat you when the new wears off. Don't let them suck you into their personal drama with their old flames. If they haven't decided how they feel about their ex, you're on borrowed time.
There's a stupid old country song that goes, "If it don't come easy, you better let it go." Truer words were never spoken.
edit: However, it's refreshing to see the number of you young 18-25 year olds who fall for the charms of old bastards. Gimme a call