You see? You just can’t trust anyone. The first girl I let into my life and she tries to eat me. — Zombieland

You have to be careful when you’re deciding to drop your defenses for someone.  Once you make yourself vulnerable to someone, not only will you be unprepared for betrayal and attack, but they will also know exactly what weaknesses you have and which buttons to push.  The vetting process should be long and rigorous, but it can’t be insurmountable.