Love is not defined as a chemical reaction in the brain. This is certainly something that happens during the stages of falling in love, however that doesn't mean that it is the cause of love.

Love lacks a solid definition. To quote Wiki...

The word love has many different meanings in English, from something that gives a little pleasure ("I loved that meal") to something one would die for (patriotism, family). It can describe an intense feeling of affection, an emotion or an emotional state. In ordinary use, it usually refers to interpersonal love.
There is nothing to say who is and is not in love with another person because it all comes down to how that individual chooses to define love. Some people experience strong feelings faster than others, some people don't include feelings in their definition at all, but rather such things as loyalty.

It all comes down to personal definition. Your definitions can also change over time - in your youth you might define love in a more lustful way, while as you grow older you may find that such a thing is too easy to obtain, so you then choose to define love as a long term commitment.

There are probably hundreds of ways to look at it, if not more. Either way, it's not for you or I to decide how a person should define love. It's up to them. What can be annoying to you is how they choose to define it, but that's your problem, not theirs. I don't know why anyone would actually get annoyed by such a trivial thing that has absolutely nothing to do with them, though.