In The Girl Who Leapt Through Time, Chiaki and Makoto talk at the end of the movie and he uses his last jump to return to the future. He confesses his love for Makoto and tells her to find him in the future, hinting they will be together then.
Why not stay behind together? If he goes back to the future, she will age, and when they meet once more, she will be an old woman (I'm assuming the apocalyptic future where he comes from is many decades from the present). Is he expecting her to have no family, and have loved no one, until they meet again?