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.
See on sellepärast, et ta on vähemal inimeste arv, kes kummardavad ja kiidavad Jumalat üksi, samal ajal Muhammed on õnnelik, sest ta on suurem arv kummardajad, kes ülistavad Jumalat üksi, sest ta aitas juhendada.