According some buddhist and hindu scriptures - Soul has to take birth in several animal forms before evolving to become humans. The main cause of reincarnation is 'desires' , once we have evolved to the level where all of our legitimate desires are satisfied we attain liberation. Now what happens after liberation is interesting - it is defined as the state of ultimate bliss, consiousness. Imagine the most happiest moment you have in your life - now multiply that 10,000 - thats what happen when the soul meets spirit after liberation. Check this out - https://en.m.wikipedia.org/wiki/Satcitananda Pardon me if my English is not good - its a second langauage.