I'm trying to think about what Sentinel's life is like post kidnapping and I think it would be pretty sad overall. Maybe not as miserable as in DW but not great either.
He may love Tarn, he may enjoy their time in berth, but it doesn't change the fact that he was ripped away from his home and is technically a hostage of the DJD.
The only time Sentinel sees his former friends and allies is when his life is being leveraged against the Autobots. Even if he managed to escape or was released, Sentinel wouldn't ever get his career back after he was forced to take calls with a blade at his neck cables time and time again during his captivity.
The DJD are pretty confident kidnappers as well. His t-cog is disabled (probably by a device that would drop him into stasis if he acted out) and there isn't a single door on the Peaceful Tyranny he can open on his own. He can't even go to the washracks without Tarn accompanying him.
Sentinel can almost pretend he's in a normal relationship when Tarn is off duty. He can let himself be flattered by the energon goodies and the care his partner takes in polishing him, but then the mask goes back on and Sentinel is stuck kneeling next to the chair of the DJD commander, wishing he was anywhere else.
Well It was like that at first. Sentinel only thinking it wasn't true love. that He was just being used, even when all the death threats stopped, and the gifts started to roll in.
I can see that he really only sees that Tarn actually loved him when if the headmaster episodes still happen and he's just a head. Tarn goes out of his way to bring him his body back without a scratch.
At first Sentinel thought Tarn was just doing it because he was his play toy- but no. When Tarn and Optimus went to go and collect his body, it was Tarn who made sure Optimus didn't do anything to Sentinels body, like ramming into it, or punching him.
"Be careful where you punch Autobot. Or it won't be this organics energon I spill tonight."
And when he gets his head back on his body, Tarn is checking him over and grumbling about how that organic ruined his paint, scratched him up.
"I'll be fine. you like me a little roughed up."
"By my own servos yes. But this... Id kill that human if Your ex wasn't so nice about putting him with his own kind to deal with."
"Come now. You really don't have to think like that. just for me."
"Why not? Should I not be concerned for my mates own well being. what if that has side affects. are you feeling any different. you must tell me if you are."
and then he's picked up, and for the first time Sentinel thinks that Tarn has lost it, and is he actually in love with him, when for the past 30 years Tarn has thought they've been together, and just wasn't fully ready to spark merge.