You will find that Sims characters have chemistry with each other which is based upon what they find to be attractive with your Sim or what you find attractive in the other Sim. Not to mention there is a jealousy factor involved if you are in a relationship and another Sim decides to flirt with you. If that happens then that causes your boyfriend or girlfriend to possibly get angry with you as well as the Sim that flirted with you. You can start fights, insult, and annoy Sims that were caught flirting or worse, doing more, with another Sim character. All of which can lead to breaking up.