When you first get into a relationship, everything is fantastic. Frequent sex, no arguments, etc.
But the longer you stay in that relationship, the less sex you have and the more arguments you have.
I'm not going to blame the lack of sex on the girl. I'm going to blame her bitchiness and arguing on the lack of sex.
The more sex a girl has, the more sex a girl wants. For men, sadly, the opposite is true, at least when it is with the same person. That's not necessarily a bad thing. Several evolutionary factors contribute to it and make it beneficial.
But for women, especially in long term relationships, the less sex she has, the less she is able to deal with stress and the more likely she is to turn her frustrations and anger towards the release of her frustrations - the man in her life.
If you're in a long term relationship and wondering why you're arguing more*, it's because you're having less sex. Fuck her rough, fuck her hard. Make her submit to your cock in bed, and she'll submit to you in life. Start early, maintain for the rest of your life.
Viagra has a greater purpose than just getting an old man off - keeping his woman happy.
*this assumes there is no real reason for the argument. If you're having real problems, this is not the answer.