The feeling of wanting someone, not just sexually. When just the thought of holding them, having them so close to you makes your heart feel warm. When they tell you their worries, problems and secrets, giving you their trust, and all you feel his happiness that they trust you and vice versa.
Someone who makes you happy just by being themselves. Just being with them, even if not in a literal sense, such as only being able to phone/e-mail them, when that alone makes you happy, that is love.