Obi-Wan and (Jedi Knight!)Anakin have to be in a fake relationship for a mission that will last months. Neither of them are happy about it.

Anakin isn't happy because he has a crush on Obi-Wan and it will be torture for him.

Obi-Wan because he's very uncomfortable with romantic relationships at all or to being that close to anyone. He's also completely unaware that he's in love with Anakin.

Of course as per the rule of the fake relationship trope they end up together.

Bonus :

- if Anakin and Padme are married, Padme is ok with this (maybe they have a open relationship and she's in Sabé's bed during all this ? maybe they fell out of love ? maybe she simply knows Anakin can love both of them ? maybe she's waiting for the inevitable Ani/Obi/Padme ?idk)
- Obi-Wan is quickly overwhelmed by all the touching and kissing and hugging that Anakin does, Anakin thinks that Obi-Wan is disgusted and is sad but the people they're trying to fool are perceptive and know that Obi-Wan is just hypersensitive (and they tease Anakin about having a husband that's so receptive to his touch which he's confused about)
- The Council totally choose the two of them for the mission because they were tired of seeing Anakin pin after Obi-Wan (and they're hoping that the relationship will stabilize the Chosen One a little)