Spirits after they die, they cross over. It is actually a procedure all of them pass and no, I don't mean like "earthbound" spirits with unfinished buisness-it's not exactly like that. After someone dies, why they all have to go under that procedure of crossing over, what is it actually and why is it needed?
Well I believe people are rencarated because my first ever memory was pepole crying over my body and saying I wish they didn't have to go like this or something then my spirit was drifting away and I started yelling cause I was scared I said wait guys I'm still her wait then I bolted up in a child's body probably about 3 and didn't know we're I was so I got out of bed and I saw this woman their then without thinking I said hi mom I just knew all of these people like my dad and sisters instantly I always tell my parents this but they always say oh well that is just your imagination but I know that was real well I hope this helped keep making theories about things it keeps you thinking
As H2OfanRikki said, we have no way to corroborate why and how souls cross over until we ourselves are dead.
Depending on who you ask they will tell you different things according to their set of beliefs, some think that it is a test that the soul must undergo in order to evolve, some think that it is the way to reach its final destination in "the other world".
My opinion is that the process of crossing over is a period of adaptation that the soul undergoes to become accustomed and accept its new life, as I believe that life does not end with death, but as I told you before, this is only an opinion and I have no way ofknowing if it is the true.