No one is capable of loving unless they have first experienced love.
Love is discovered and learned experientially.
Every time a child runs to mother with a scraped knee and receives heartwarming comfort love is being taught.
This may be so obvious it is easily overlooked so let me say it from a different angle.
If traumatic events, especially in childhood, are never responded to by anyone in loving ways the child grows up ignorant of the power, even the necessity, of love.
This means unloving people were often in the past unloved or poorly loved people.
Do not take this as giving a pass to poor behavior, but also, and more importantly I think, is to understand the power and importance of loving acts.
Also there are many life examples of people who have grown up in horrific circumstances who still became wonderful loving people.
How can this be?
I submit to you that they found love somewhere, and directly by God should not be ruled out.
As for you, O LORD, you will not restrain your mercy from me;
your steadfast love and your faithfulness will ever preserve me!