On two occasions, three if you count Kess, the Doctor has given up his life in order to save - or attempt to save - a companion from their certain death. The Fifth Doctor saved Peri by giving her the batsmilk tonic to cure her Spectrox poisoning, allowing himself to succumb to the same illness, prompting his regeneration. The Tenth Doctor, knowing that hearing four knocks would signal his demise, chose to rescue Wilfred Mott by allowing himself to absorb the radiation in the dual sided chamber, while freeing Wilfred from the adjoining one. The Eighth Doctor went down with the ship that Kess was attempting to pilot, choosing to stay with her and talk her into being saved by a Time Lord, versus running away (which he could have done, being right next to his TARDIS). In all three occurances, these were people the Doctor hardly knew yet chose to save and force a regeneration upon himself, versus being able to easily save himself in all three situations. Lesson Learned: The lesson here isn't to allow yourself to die to save another, though there are some that will do that, including members in the military all around the world. Instead, help those in need as best as you can, even if you don't know that person that well.