Starwars episode 4?

why during the saber fight between obi and darth did obi just seem to give up towards the end he looks at luke and back at darth and then sort of smiles and lifts the saber to his head and darth of course attacks and he is gone... why did he do this??? was it just cause he knew it was over for him and that he could never beat darth or was it cause he knew he would be a better use for luke by becoming part of the force in spirit?