There are a lot of ways, but the biggest one is just a sense of feeling connected to each other and knowing each other on a deep level. You may also feel as if you are meeting yourself in another form or that you are too much alike, so to speak. You may have similar childhoods, good or bad, which means you mirror each other's trauma on a weird level. You may fight a lot or trigger each other but still want to be with each other. Lastly, you leave the relationship wanting to grow as a person, not necessarily for them but for yourself. You probably won't know who this person is to you until after you've split, sometimes years later.