Since the beginning of time, people have had someone that completes their soul, that person is called a soulmate. Every once in a while, you can see from the perspective of your soulmate. It happens at random.

Optimus had come to terms with the fact that he'd never have a sparkmate, as he was never forged with a designation etched on his arm plating... until one solar cycle, when he awoke from recharge and went through his usual duties as a prime and leader of the autobots.. and Ratchet pointed out something there. At first, he imagined it was some sort of energon stain until he looked at it and saw.. a name. He saw a name. Why now? He was a Prime, he was supposed to be dedicated to his people... but.. he felt excited now. He wouldn't show it, Primus no, but he felt happy..

He didn't understand when their perspectives would swap, why he was seeing.. organic things. Not until he reached earth. After a while of existing on this planet, he realized that.. this is where his sparkmate must reside.