Starcrossed

This is missable because there's only so many opportunities in a playthrough to curry favour with your active companion. So pick one to stick with, and do/say things that they will like. Whenever their 'activities' to talk to them pop up in your journal, do so. Do the companion questline.

Eventually your relationship level will rise enough that there'll be a marriage or ceremony - the companion will signal that its time. Go along with this and on completion the achievement will pop.

Don't panic about reaching 'Friend' status - you haven't been friend-zoned, this is just a step along the way...