5. All is fair in love and war

In love and romance, you sometimes do things that aren't very nice. For example, you might break up with someone or steal another person's partner. This phrase means that that's OK

Nothing is out of bounds when it comes to love and war. The rules of hard play are acceptable in war. In someones eye's "all is fair in love and war" in other words, in a war between love and hate, anything is acceptable or fair.
The only two areas of life in which you can be forgiven for doing anything are love and war. For example, if someone kills one of their relatives on the battlefield, it is more acceptable than murder usually. Similarly, stealing a friend's lover is seen as more acceptable if you love them, instead of say, just wanting them for money.