Lorenzo St. John spent most of his long life unwanted and unloved, abused or abandoned by everyone he met. This was true for his time as a human, as well as after he became a vampire. He never thought anyone could ever care about him the way he wanted so desperately; it didn't stop him from hoping. 

He thought he'd found someone in the '50s when he met Damon Salvatore in the Augustine prison cells, but his friend betrayed him, left him alone to burn alive. For several more decades, he was tortured and experimented on with no end in sight. It was a miserable experience. 

Enzo never thought that he would find someone who would choose to stand by his side, someone who would love him as he loved them. But eventually, he was proven wrong by one stubborn witch. Bonnie Bennett was the most passionate, loyal, determined woman he'd ever encountered, and he was charmed by her instantly. Love came later, but it did come. What was astounding to him was finding out that Bonnie loved him, too. For three glorious years, she was his entire world, as he kept her safe from those who wanted to use her power for their own gains--as so many had before. All he wanted was to protect her. 

Eventually, they decided they wanted a real life together. So Enzo St. John took the cure and became human again. It was, admittedly, a little frightening at first. He'd spent so long relying on his superior strength and agility to take out his enemies, and being a human left him feeling a little defenseless. But the tradeoff--getting to grow old with the love of his life--was more than worth it.