The chazan is the emissary of the congregation, and therefore a person is prohibited from taking hold of the chazanut unless he is asked to do so by the congregation or by the gabbai as its representative. Hence, one may not respond Amen to a person who appointed himself to be chazan against the congregation’s wishes (Rama 53:22).
When the gabbai asks someone in the congregation to lead the prayer service, it is polite to initially decline, so as not to appear as one who desires to flaunt his voice. However, when the gabbai insists, he should prepare himself to ascend, but wait to see if there is another person more appropriate than he. If he is asked a third time, he should comply and begin to pray. However, if an important person asks him to ascend, he should accept immediately, for it is not proper to refuse a distinguished individual (Shulchan Aruch 53:16). Additionally, when the gabbai asks a person who declined in the previous prayer service to ascend, he should consider it a second request and ready himself to stand, but wait to see that there is no one more appropriate. If asked again, he should concur immediately.
If a person is able to lead the prayer service but declines more than the Chachamim instructed, he offends the respect due to the prayer and to Heaven (kevod Shamayim). Likewise, a person whom Hashem endowed with a talent for singing and a pleasant voice should not decline on Shabbat and festivals since the prayers on those days are rich with song and melody. If he refuses to pray out of stubbornness or laziness, and does not praise Hashem with his voice, it would have been better for him not to have come into this world (Sefer Chassidim 768). The Chachamim said about Navot the Israelite, who had an exceptionally pleasant voice, that he would ascend to the Temple in Jerusalem for the three pilgrimage festivals (shalosh regalim) and all of Israel would gather to hear him. The one time he stayed home to guard his vineyard, he was punished – lawless people testified that he rebelled against the king and he was put to death (Pesikta Rabbati 25).