Jacob can appear to be quite a generic one dimensional space marine type with his typical boy scout demeanor and a need to protect those weaker than him. He is quite a difficult character to like because of these traits, he appears to have no shades of grey.
He does hide a needy interior and loves being in service of Shepard and is in complete of awe of his/her military prowess. There is a certain side of him that needs validation of achievement through the protection of others. Through the course of the game though, he has shown himself to be quite conflicted -- as he only pretends to love the fast lifestyle of being single, and serving a military life there can be a certain level of bravado to his character.
Deep rooted father issues which are revealed in his companion mission shows that while he tries to be smooth with the women, he clearly needs to feel wanted even just for a brief moment in time. This is a reflection of why he cheats and sleeps around. Jacob eventually reveals that he would love get serious and start a family.
His romance options aren't the most fondly revered amongst fans though and is perhaps more memorable for his comparisons to the "old spice man" and his remarks about a certain "prize."