Asking "what is love" is like asking "what is swimming". Both refer to clear actions.
Swimming is the action and behaviour of moving through water using your limbs as propellants. It manifests itself differently in everyone. Some people do breaststroke, some do the backstroke, some stick to the doggy-paddle, but all are swimming.
Love functions in the same way. It is a verb. To love someone is to long for and display immense affection for another human being. Like swimming, this manifests itself in different actions, but all are encompassed under the verb 'to love'. The key aspect is that it is an action. This view of love also makes love quantifiable. If doing XYZ means someone is in love, and he or she does these actions more with person A than with person B, then it is fair to assume that he or she loves person A more than person B.
Love cannot be reduced to behaviour. Our definition of love must include more. If love was simply the way we behave towards other people, then actors would be in love with their co-actors or actresses when playing the part of an on-screen couple. They are behaving as though they are in love, therefore, by this narrow definition, they are in love. Love must be more than simply acting or behaving in a certain way. It must include feeling and desire. The action without the presence of the feeling is not love, it is acting.
[P1] Love is a verb. [P2] Verbs are action words. [P3] Therfore, love is an action, much like swimming.
[Rejecting P1] Love is also an abstract noun. [Rejecting P3] Love cannot be reduced to actions. It has a corresponding feeling which is necessary to be in love. Without the feeling, you are acting as though you are in love, without being in love.