Asking for someone else. Rashi on the parsha, Beresheet 16:3, brings a midrash from Beresheet Rabbah 45:3 which says:
"And Sarai, the wife of Abram, took Hagar the Egyptian, her maidservant (Genesis 16:3) - took her with words, saying "you are fortunate to be clinging to such a holy body like his!""
Previously she had been a maidservant with little chance of this status changing - why wouldn't she jump at the chance to become one of the mistresses of the household? Why would Sarah need to convince her to do it?