What is swimming?
Asking "what is love" is like asking "what is swimming". Both refer to clear actions.
Love is not something passive that happens to us. We have a choice and we choose to love someone.
When two people meet and “fall in love”, in the beginning, they think about each other constantly. They want to spend every waking minute with each other. They shower each other in gifts and pine after one another. But these feelings are fleeting. They fade. Keeping those feelings alive and staying in love requires a concerted effort. It is not passive. It is not simply something that happens to us. We have to actively work at it.
Staying in love requires work. It means setting time aside to spend time with your loved one. It means accepting their personality flaws and looking past imperfections and choosing to love them regardless. To reduce love to a sensation, or a brain process, would be to make the agent (the lover) passive in the process when they are very active. Love is behaviour. But it is also more than that, it is making a decision to stay in love and acting on it. This applies to all kinds of love. A parent makes the conscious decision to love their child. There are times when the child may misbehave, disappoint, or embarrass. It may push the boundaries of acceptable behaviour throughout childhood and into adulthood but the loving parent chooses to give their love unconditionally or chooses to rescind it.
This is not always true. We don't always have a choice. People do sometimes fall in love with someone they don't want to fall in love with or aren't supposed to. The whole Romeo and Juliet love story is based on a forbidden love that cannot be controlled or stifled. We don't always have a choice in love. Therefore, there are times when we are a passive agent. Any definition of love must adequately incorporate and explain these instances of uncontrollable and unexpected love.
[P1] Love is a choice. We have to actively make the choice to be in love. [P2] Therefore, love is not reducible to a passive feeling or brain state. [P3] Therefore, love is our behaviour. It is the way we act towards another person.
[Rejecting P1] Love isn't always a choice.