Without disagreeing with the above, I should add a few points.

First, Marlo is seen with the occasional hot girl. In season 3 Stringer and Avon use a woman to lure Marlo, (though Chris catches the trap and the girl is killed).

Second, Marlo does show interest in material things - I do believe that he and the aforementioned girl/bait have sex leaning up on his Escalade, that vehicle fairly pimped out.

Thus, Marlo is not a sociopath devoid of any discernible motivation. He likes his rims and his girls. Marlo can certainly be defined by his sociopathic tendencies - they are his most prominent feature. But he is also something of a sensualist, seeking thrills and possessions to fills what is otherwise an emotional void.