Ultimately, the reason why love and compassion bring the greatest happiness is simply that our nature cherishes them above all else. The need for love lies at the very foundation of human existence.
Those who don't have a life filled with luxury may have a home filled with compassion, based on their choice to be content and to practice self-discipline. Even when we have physical hardships, we can be very happy.
If indeed the qualities such as love, patience, tolerance, and forgiveness are what happiness consists in, and if it is also true that compassion, defined as concern for others, is both the source and the fruit of these qualities, then the more we are compassionate, the more we provide for our own happiness.
A human being is a part of the whole called by us universe; a part limited in time and space. He experiences himself, his thoughts, and his feelings as something separate from the rest - a kind of optical delusion of consciousness.
Supreme good is like water. Water greatly benefits all things, without conflict. It flows through places that people loathe. Thereby it is close to the Way.
It is not enough to be compassionate. You must act. There are two aspects to action. One is to overcome the distortions and afflictions of your own mind, that is, in terms of calming and eventually dispelling anger. This is action out of compassion. The other is more social, more public. When something needs to be done in the world to rectify the wrongs, if one is really concerned with benefitting others, one needs to be engaged, involved.
There is a common perception that compassion is, if not actually an impediment, at least irrelevant to professional life. Personally, I would argue that not only is it relevant, but that when compassion is lacking, our activities are in danger of becoming destructive. This is because when we ignore the question of the impact our actions have on others' well-being, inevitably we end up hurting them.
If once we admit, be it for a single hour or in a single instance, that there can be anything more important than compassion for a fellow human being, then there is no crime against man that we cannot commit with an easy conscience.