The word 'soul mate' is a very general category for a variety of perceived ride or die relationships.
On the occult level, it can deal with two being 'as one' by a very entrenched bond, yet this isn't the same concept as what I said in the first sentence because people's perceptions can be really off. Others get confused here and think this requires marriage or the most traditional type of requirements which isn't true.
This is a matter of a lot of complex factors.
Astrology shows how fated this is, but that's all it is, fate. Fate in itself does not have a positive or negative quality. Yes, two can be yoked together so much that fundamentally, they almost become one. A lot of this deals with Pluto. When two people are bound together on the most profound way, it can have very negative as well as positive manifestations, under the enemy anyway.
Scorpio is the sign that rules over a lot of this, but so does Libra, and these signs together are called 'the Claws' for a reason.
Soul mates are typically an ACCUMULATION of Synastries that evolve with intense effort, struggle and sacrifice throughout lifetimes between two souls, and sometimes this can go backwards and forwards. This is also how two perfect enemies can develop.
The idea these types of bonds are at once created by a random glance in the hallway is a specifically jewish mythology.
The other thing is that the average person - even when married to someone else for seven decades - was not, is NOT with a soulmate. Sorry if all your GI gen great-grandparents and the cute romance movies like the Notebook and Love Story told anyone otherwise. Most people do not marry near this and their romantic bonds simply end with their lifespan. HP Cobra said something to this effect lately that the bond wasn't as important as many people think.