Because of Lucky by Ruby Sparks

Kevin McMurray never imagined he’d run into his old crush/heartbreak after twelve years. And it’s all because of a stray dog he rescued from the streets.
It took Kevin twelve years to forget Gale Harrington, the man he both loved and loathed. Now that was all about to be undone. Lucky, as he’d named the German Shepherd, needs a place to stay until his people can be found. Kevin is up for the task, but Lucky is a link to Kevin’s past he doesn’t want. He can’t say no to an animal in need, however. Gale reveals things and before Kevin knows it, he’s falling for the guy. But Gale is dangerous, and Kevin has no idea what the man wants from him.
All Gale ever wanted was to apologize to his former best friend for his crappy behavior in high school. He never thought he might get the opportunity.
After some personal discovery, he is happy where life has led him. He loves his job as a veterinarian at the local animal shelter, and owns one sweet crash pad. But when Kevin walks back into his life unexpectedly, he realizes something is missing. His best friend. But saying the words are harder than he thought. Thankfully, Lucky gives them a reason to reconnect. As the prospects of locating Lucky’s people grows dimmer, his bond with Kevin strengthens into more than just friends.
But can Kevin get over the hurt Gale caused?