What constitutes love?
If you ask a youngster, it's hormones. It's when you are addicted to another person. They make you feel so good, you feel like you're not alive until they plug you in. If they spend more time talking about how the other person makes them feel, they are likely in lust.
If you are a young adult, it's responsibility. It's when you feel that someone is a safe partner to breed with, to make a nest with, to plot an old age and retirement with in the future. It takes some chemistry to want to make children with someone, but it also takes some practicalities, like income potential, security, and feeling safe.
If you're middle-aged, it's companionship. After knowing lust and knowing practicality, the middle-ager learns that ultimately sex goes only so far and security only a little bit further, but until there is some shared interests and dreams, the time together is purely having sex and doing chores and otherwise ignoring each other.
What love is...
You love a person when you admire his character.
You love a person when you trust him with your greatest secrets.
You love a person when he shows you through deeds and words that he has a heart.
You love a person when you would like to be more like him.
You love a person when you wish for him to meet his dreams and want to help him get there.
You love a person when making him smile or laugh is the single greatest currency you have ever earned.
You love a person when, even if it is not you that gives him fulfillment, you wish him the best on his journey.
Ultimately, to be in love, your focus is on your love's heart and keeping it safe more than your own.
What love is not...
Love is not obsession for another person to the point of giving up all your activities and friendships.
Love is not something doled out only in good times and withheld during the bad.
Love is not under the conditions of i.e. perfection, achievement, acquiescing, or sacrifice.
Love is not a habit or an obligation, it does not run on autopilot and cannot withstand neglect.
Love does not incite hate, anger, retribution, or revenge.