Re: Why do we die?
Well … 
Dear brother, according to my knowledge, Allah created all souls at the same time in distance past … and also told humans many things … and souls also promissed to Allah many things, that they would abide forever, in all circumsances.
Phase 1: Since that day (day soul got created) to the day one is born in this world.
Phase 2: When soul are born in this world, they are made to forget (or put into their subconscious rather than conscious) a lot of past so that they can get tested in this world. Birth attaches soul with body through Nafs (as glue). Thus life in this world.
Phase 3: Once we would leave this world, or leave the body we have, or that our Nafs would die and thus link of our soul would get disconnected with our body, soul would again get back to ‘the world of souls’, thus life after death but before day of judgment.
Phase 4: On judgment day, our soul would get a body again (in what shape and form we do not exactly know), and that would be a phase of its own.
Phase 5: After judgment, our soul would reach a place of ‘bliss’ or ‘gloom’, in what form and shape we do not exactly know, most likely we would have some sort of body, and that would final phase … or is it?
I mentioned the phases that I know of (or at least according to my knowledge and belief), but only Allah knows the best.
Anyhow, I fear that there could be another phase waiting for us, and that could be:
Phase 6: It is possible that in far future, Allah may decide complete annihilation (extinction) for most souls, replacing us with new creations … or selecting some from us to live and ending existence of others … who knows … and that would be end time for our soul to exist that can be called ‘existing forever’ as far as our soul is concerned, as ‘forever state for soul’ mentioned in Quran could mean 'until soul exists’ (be that in heaven or hell).