There's a fascinating development in our understanding of ourselves in the works, presently. It backgrounds this notion of 'soulmates.' You can find a brief overview at this New Scientist reference:
and, if you want all the information, you will find it in the journal referenced in the article.
A research team led by Uri Hasson of Princeton University, scanned the brains of subjects as they listened to a storyteller relate a story. The scans revealed that the brain patterns of the listeners tracked the brain patterns of the storyteller.
The pattern harmonisation did not work though, when the story teller related the story in Russian. Hasson stated that the brain patterns of the listeners became 'intimately linked' in this experiment.
This experiment probably explains not only the soulmate idea, but also the mass hysteria that can affect large groups in the sway of a hypnotic speaker, and yes, the 'motivation' felt en masse at meetings and assemblies (unless the speaker was brain-bonked) and religious services such as pentecostal serives.