The Preacher has held up nearly everything under the sun and watched it slip through his fingers like vapor. Pleasure, wealth, accomplishment, and even wisdom,  he has chased them all and found each one unable to satisfy. He's looked at the oppression around him and the tears of those with no one to comfort them. He's watched people work themselves ragged out of nothing but envy of their neighbors. Just before our verses, he describes a man toiling away utterly alone — no son, no brother, never satisfied, never once stopping to ask "for whom am I working?" He calls it an "unhappy buisness". 

But there is something he is not skeptical about. "Two are better than one... For if they fall, one will lift up his fellow. But woe to him who is alone when he falls and has not another to lift him up." He uses several pictures, working, falling, the cold of night, facing an enemy,  naming every basic vulnerability of life and answers each with a companion. Solomon, who has experienced much and found it empty doesn't doubt this: you were not meant to be alone.