It's been literally 5 million years since he's seen you. You were lost during a scouting mission on Cybertron during the war. Everyone looked for you, knowing your importance as his wife/Conjunx. Everyone searched for solar cycles on end but couldn't find you. He was devastated. When you "died", so did his happiness.

However...

The Autobots had retreated to Earth after Cybertron had fallen. There, they built a sanctuary to keep themselves safe from Decepticons and humans. Just as Optimus went for a walk, still missing them deeply even after all these years.. that was until he noticed a small glimpse in the distance.. his spark raced and hope swelled up, even though he knew your return was impossible.. right?..