Because God created us for relationships (Gen 2:18; Acts 2:42-47; Heb 10:24-25; Ps 133:1; Matt 18:20). God Himself said, "It's not good for man to be alone" (Gen 2:18). Before Eve, Adam was alone. Yes, he was in perfect relationship with God, however God wanted Adam to be able to express love and experience what it is to be loved by someone like himself.
Because God commands us to love others (Matt 22:35-38; John 15:13; Rom 13:10; Eph 4:2; 1 John 3:16-18). Just as God did not see it fit for Adam to be alone, so too Jesus calls us to love others. He commands us to love God and love others (Matt 22:35-38). We cannot love others unless we engage them, meet with them, dine with them, share with them. Small groups create one of the best platforms for this to take place.
Because spiritual growth happens best in the context of community (Prov 27:17; Ecc 4:9-12; Gal 5:16-26). "Just as iron sharpens iron, so one sharpens another" (Prov 27:17). There are 59 verses in the New Testament where God is calling His children to "one another!" Love one another; forgive one another; submit to one another; build up one another; confess to one another; spur on one another; pray for one another. It seems quite clear that God is calling us to each other far beyond meeting once a week on Sunday morning for merely one hour. Small groups are an extension of the church, yet are able to reach deeper and more intimately in ways that the church cannot.