They are insecure and need to control every aspect of love, or they fear it will slip away from them in the night. In such people, jealousy is an ever-present monster. If they don't feel it themselves, they attribute it to others. More relationships have ended because one of the partners blamed jealousy for problems that were completely separate from that issue. Love that is limited, controlled, defined by only one partner, or conditional, is not love.
True love is expansive.
The more we love, the more we expand our ability to love. Many speak of the chest area holding a heart chakra. Whether this is a fact, or merely a way to describe the feeling in the chest as the heart opens, will always be a matter for conjecture. What will remain constant is that to love, you must love. And to truly love, you must love equally.