my throam (maybe) hot-take
The Heart Rate of a Mouse is not primarily a love story.
I couldn't figure out why it gripped me so much as a teen but reading it again now that I'm old it's kinda obvious.
It's a story about finding a home.
Ryan is a fucked-up kid from an upbringing that made him think he could never really be loved by anyone who knew him. He had this hope that he'd found a home with Mrs Roscoe but it got shattered and he gave up looking for so long because he thought that was his only chance.
Brendon is a lost kid who had to leave his home and is desperate for the security of it again. He gets so used to being transient that he's willing to settle for Shane just because he thinks that's the best he can get.
And neither of them can let themselves have each other because they're so broken they can't believe they're allowed it. They can't trust it.
But they figure it out and they build their family in Jon and Sisky and Spencer and Vicky they finally accept that they can both 'try to have one'. And Ryan does build his home with a Roscoe, just not the one he expected.