There is a reason Leigh Brasington teaches loving kindness as a preparation practice for the jhanas. Unless the heart is fully opened, and unhindered, the mind cannot fully settle. And a disturbed mind bars one from the possibility of inner peace and freedom.
Loving kindness is a benchmarking practice for the heart. It shows us how many obstacles the mind sets to love flowing out freely. It also bumps against automatic responses from a body most often used to closing up.
Sitting, practicing loving kindness, I get to see all the idiotic reasons perpetuated by the mind, for why I should not love such and such person.
"I hardly know him. Love is for close loved ones, ins't it?" Mind operates under the delusion that love is a limited resource, to be allocated parsimoniously only under certain conditions.
"He has hurt me. I cannot trust him. I don't feel like loving him." Mind makes gigantic leaps, from needing to be cautious, to slamming the heart door . . . Not realizing that each slam is a source of stress for the whole system. I am not hurting him who has no idea of the movie playing in my heart. I am hurting myself.
"She does not seem to care. Why should I love her?" Mind keeps on dispensing evaluations. Lovable? Yes, a little, or maybe a lot, or not at all?
Turning towards the heart, I get to see all the conditions set up in the mind only, and that keep me from finding the joy within.