Relationships can be devastated by simple, thoughtless things. Bad behavior will ruin your marriage faster than any temptation from the outside. Tina B. Tessina, PhD, "Dr. Romance" licensed psychotherapist and author points out the worst mistakes to make in a marriage.

Dr. Romance's 3 ways to ruin your relationship:

1. Pick the wrong partner for the wrong reasons: No matter how charming your partner is, if he or she's a player, an out-of-control spender, a con artist, an alcoholic/addict or violent, no amount of love on your part will fix him or her one bit. Don't try. The minute you find out there's a Fatal Flaw, end it. Find a less charming, but more upstanding, healthy person to love.  

2. Nag/scold/bitch/yell when things don't meet your expectations. You have to take care of yourself, and find a way to solve problems and motivate your partner to work with you. Partnership is the name of the game, not "I want you to take care of me, and I'll throw a temper tantrum if you don't." You'll get a lot more of what you want if you ask directly and simply, and motivate with affection, humor and fun.  

3. Do it all yourself. Lots of people try to fill in all the gaps by doing whatever their partner isn't doing -- all alone. If he can't keep a job, getting successful on your own could be a good thing for you, but it won't save the relationship. If he won't help around the house, or with the kids, doing it all yourself (plus your job) won't save the relationship either. If she won't be responsible about money or discipline, doing it all yourself will work for a while, but you'll wind up being seen as the bad guy. Very early in the relationship, give your partner the room to do his share. If nothing is forthcoming, ask directly (don't just whine or hint) for what you want. If your mate doesn't step up, and won’t discuss what would help, then you're probably the only one in the relationship, and it's not going to work.

(From Money, Sex and Kids: Stop Fighting about the Three Things That Will Ruin Your Marriage.)