Because he wanted to be part of the force to help luke.

because it was lukes destiney to kill darth vader

Because he wanted the rest of the team to make a clean getaway without having to go back for him, since his job of keeping Vader occupied was done.

Because he wants Luke to hate Vader so that he'll one day kill him, finishing the job Kenobi started on Mustafar but didnt' have the guts to finish.

I think it was because Kenobi realized that the new Jedi would have to take a different path than the old Jedi did. After all, look how much ass the Jedi in the prequels kicked. Mace Windu lopped off Jango Fett's head right in front of his kid! Those Jedi were bad-asses who didn't hesitate to cut down their enemies. And look what happened to them. They grew so far out of balance because of their shortsightedness and power in the Republic that they didn't even see the Sith in their midst. All their battle-prowess couldn't save them in the end--it just made them more vulnerable to the powers of the Dark Side. That's what Kenobi wanted to show Luke. That being the strongest and the most powerful isn't what it's all about. It's about doing the right thing, and being calm, at peace, passive. The Jedi cannot win by strength and conquest. Jedi can kill anyone, but they shouldn't, they don't have too. They should hold themselves to a higher standard. If you can squish someone's head with the Force, you can take their blaster away and mind-trick them into surrendering just as easily, and that way you don't leave behind any grieving orphans and widows to become the next generation of ruthless vengeance-driven bounty hunters with a mad-on for your hide. Yoda said war does not make one great. He and Kenobi had to lose an entire Jedi Order to learn that lesson. It was worth any price to make sure the first of the New Jedi Order would learn it, too, and pass it on, so the fall of the Republic wouldn't have to happen all over again.

Why not remember what he said "If you strike me down I will be more powerful than you ever imagined"

I read an interview with Alec Guiness a few years ago. He said that he HATED working on Star Wars, that he felt the lines he had to recite were insipid and silly, and insisted to George Lucas that his part be written down to a bare minimum. He regretted having taken the part. So they killed him off so they could minimize his role legitimately. Frankly, I loved his part. I think he was wrong. But who's to argue with him?

because he had mastered life after death,after being killed he came back in the form of himself as a ghost using the force(as the jedi code says"there is no emotion;there is peace.there is no ignorance;there is knowledge.there is no choas;there is harmony.there is no passion;there is serenity.THERE IS NO DEATH;THERE IS THE FORCE.")so he thought,I could die,come back more powerful than ever(as far as he is unable to die,because he is already dead),I can teach luke along with yoda and make him(luke) end this once and for all.

he sacrificed himself so the others could escape, knowing, as Vader did not, that death would not be the end for him.

he didnt die he evaperated into the force to make him stronger this did not kill his soul if you want to put it that way but he was still alive with the force

If you remember what he says right before Vader kills him - "Strike me down and I will become more powerful than you can ever imagine" In the prequels, it's obvious Anakin does not understand the Force the way Obi-wan does. By becoming "one with the Force", Obi-wan transcends the level that Vader and everyone else is living on, and becomes omnescent, in a way. Not sure I am explaining myself very well, but it's sort of like when a Buddhist reaches Enlightenment kinda thing. I reccomend a book called "Star Wars and Philosophy" which can explain questions like this much better than I ever could!

Vader would have won. So Obi-Wan prepared himself to join the force rather than be killed.

By doing so it made obiwan more powerful than ever.

well, Lucas intended it to serve as a distraction to allow the rebels an escape, but - as with many things in the star wars universe - that's not logical reasoning and it just made it confusing. p.s. i like locutus' as a cover-up explanation.

He said why right at the beginning. "I'm getting too old for this sort of thing'.

Obi-Wan let Vader kill him because... Kenobi's death would be the only thing that would emotionally push Luke to want to become a Jedi. Luke was an emotional wreck since his aunt and uncle died and everything was new to him, this whole Force deal, so Kenobi knew that he must affect Lukes feelings in order for Luke to progress in the ways of the Jedi.