Jamieson, Fausset and Brown on 2 Corinthians 1:3

3. This thanksgiving for his late deliverance forms a suitable introduction for conciliating their favorable reception of his reasons for not having fulfilled his promise of visiting them (2Co 1:15-24).

Father of mercies--that is, the SOURCE of all mercies (compare Jas 1:17; Ro 12:1).

comfort--which flows from His "mercies" experienced. Like a true man of faith, he mentions "mercies" and "comfort," before he proceeds to speak of afflictions (2Co 1:4-6). The "tribulation" of believers is not inconsistent with God's mercy, and does not beget in them suspicion of it; nay, in the end they feel that He is "the God of ALL comfort," that is, who imparts the only true and perfect comfort in every instance (Psa 146:3, 5, 8; Jas 5:11).