1) If you ever lie to me
Honesty is one of the key elements to any relationship. I trust that someone is telling me the truth until they give me a reason not to trust them anymore. And once that happens, that trust will take forever to rebuild. If ever at all.
2) If you ever yell at me
I've had too much experience with guys looming over me, demanding this or that, and thinking that raising their voices at me will make me “listen better” or “pay attention” or that that's the best way to get a point across to me. But it has the opposite affect. I lose respect for them. I get scared. And above all, want to be as far away from them as possible.
3) If you ever manipulate me
I don't think girls realize how easily guys tend to manipulate us. It took me a while before figuring out that the way a certain guy would talk to me, and ask me things, or try to get me to do things for him, were all forms of manipulation. The second a guy says or does something to make YOU feel bad, it's manipulation for something that he wants you to do. I've had too much experience with this and am very aware now of when it's happening. Thanks to a few jerks I've had the pleasure of knowing.
4) If you ever give me a reason to not trust you
I would never be in a relationship with someone I didn't trust. And by trust, I mean, trust in and with anything. Money, girls, my belongings, my family.
5) If you ever hurt me
I don't know which is worse – physical or emotional abuse. I don't think either can be weighed in a spectrum of painful damage. I've experienced both and I can guarantee that if either happens just once, you won't get a second chance.
6) If you ever stop pursuing me
This may sound like an attention-seeking, childish reason, but in all honesty, there's nothing worse than falling for someone, going through the honeymoon stage and then one day, waking up and realizing that they're “used to you”. If you truly love someone, you will pursue them indefinitely. You won't take them for granted. You won't assume that they'll just always be there. They're choosing to love you, as you should choose to love them. And show them this love. Even if you aren't a super heart-to-heart, oozing feelings type of person, there are always ways you can show your significant other that you appreciate them, and love them in the way they need to be loved and how they need to feel valued.