In Greek there are two different words for Love.
Agape is the most innocent sort, what we feel for our children, our family, our friends.
Eros is Love cum Passion or Sex. Of course you can feel both for your companion.
When we are in love, however, we use the word Eros, we are in Eros.
When we make love, sex that is, we make Eros. Eros is the imperative need for lust and pleasure.
Eros will burn you, Agape will redeem you.
Eros denotes something stronger, at times more superficial; Agape is more tender, not so loud, can last longer.
In Greek a mother calls a child "Μy agape", My love, and a lover uses the same word for his or her companion. Agape in this context indicates both kinds of Love.
The proverb «All is fair in love and war» should then be «All is fair in Eros and war» and, with your permission, I would like to complete: «Because Eros is the most cruel war of them all».