It is because he will have a lesser number of people who worship and praise Allah alone, whereas Prophet Muhammad is happy because he will have a greater number of worshipers who praise Allah alone because he was instrumental in guiding them.
Waa sababtoo ah waxa uu yeelan doonaa tiro ka yar dadka caabuda oo ku ammaanayaa Eebe kaliya, halka Nabiga ku faraxsan yahay, waayo, wuxuu yeelan doonaa Tiro badan oo dadka cibaadaysanaya oo ammaanaan Eebe kaliya, sababtoo ah wuxuu ku ool ah isla markaana hanuuninaya.