On the Buddhist path we generally treat opening the unconditional heart as a process. This is because the love and care we show up with in the world is an act and not a product. Consciously entering a process, we give ourselves time to nurture and develop through consistent intention setting and familiarization.
As with most transformational work, conscious intention also plays a large role in how we grow our loving heart. This is because unlike motivations, which can be both conscious and unconscious, intentions are always conscious. When we set a strong intention to develop love unconditionally, we are choosing the challenge of valuing and caring for (at least in thought) every single being we come into contact with, whether they are offering us something useful or not.
First, it's important to reflect on why we would even want to develop an open and unconditional loving heart in this way. Here, if we start to look around us, beyond our familiar social circles, we will begin to see that just as we wish for happiness and wish to avoid suffering, so do all beings. Deepening this recognition can serve as a base for coming into an experiential understanding of not only our own deep inner worth, but that of all others.
Reflecting on this daily we will eventually start to see strangers as friends and challenging or difficult people as new and curious opportunities for practice. We may start to recognize that our constant and nagging self-absorption is our only true enemy, as this is often where our suffering begins.