Certain people, animals, places, events easily open our hearts, so that we, as individuals, essentially disappear, transforming into loving energy, freely, selflessly flowing into, nurturing our loved one. This is where empathy, respect, forgiveness, gratitude and joy also happen.
Other people, animals, places, events easily trigger self-concern. When "me, myself & I" dominates, our hearts are closed unless there's something in it for me. Then a bit of "conditional love" sneaks out - but it's essentially a business transaction. Fear rules here: "taking care of #1", "greed is good", "an eye for an eye, a tooth for a tooth", cynicism, restlessness, revenge, depression, anxiety, loneliness, paranoia.
It can take a lifetime to realize that "taking care of #1" can't ever truly satisfy. As we mature, we gradually realize that we only feel right when we are being egoless unconditional love. Our true nature is unconditional, transcending words & concepts.