you love him so much,why didn't you ask him to stay?
或：you love him so much,why didn't you let him stay?
you love him so much,why not letting him stay?
Why not asking him to stay as you love him so much?（三楼改得很好!在这里,从句中as有既然之意）
（用why not句型可省略一个you,意思更流畅）
一、四楼的leave是双义词,既有“离开”也有“让、留下”的意思,有歧义,所以最好不要用……