What is LOVE ?

Love. What is love? How do you know when you love someone or to say that you actually mean it? Is it when  you know deeply in your heart that you belong with that person? Is it that person you would do anything for? I've felt as though I thought i loved someone and realized that it was just the feelings going to my head. Or that impulse of just wanting to have someone there to have. When i watch movies and things the kind of love i see is the rare love that only comes to the lucky people. They don't have that doubt in their mind of maybe i love them or maybe i don't they just love without thinking and when i watch it it's the passion that makes you want to love or to be loved. The moment of feelings between two people who can't stand to be apart or that when you are with them you feel like time is frozen and you wish it could last forever. Or that first intense kiss that when you close your eyes all you see is stars and fireworks. Yet just the site of them gets you excited or that you have butterflies in your stomach. Also the unconditional love. The love where no matter who you are or who you used to be that the love you for you perfections and even for your flaws. The trust of knowing they'll be there for you no matter what. Or even more important that they will spend their whole life waiting for you even if it means forever. That they see more than just a face to look at but a person to love and to be with. Someone to grow old with that to you will still look as beautiful as when you first met them. To know that you have someone to be there to catch you at times when you fall. Yet someone who can also create their own mistakes and some how you still love them. So what is love ? Where does it come from ? How do you get it or how do you know your getting it in return? Bt how do you know you really LOVE someone?