It could be instinct, yes. Instinct of survival is one of the strongest instincts that any living human or animal possesses. However, if you take into consideration the WILL to live, it will conflict with the INSTINCT.
Also, perception is everything. For example, imagine a single, 40-year-old man with a dead-end job who moved away to a state in the US where he doesn't have any family or friends yet. Let's say he moved out there because his job, so he felt he had no choice but to leave everything behind. This can create a feeling of loneliness. And with no spouse or children at that age, he can easily think, "Man... What am I doing with my life?" Life can get difficult to live.
BUT if he changed his PERCEPTION of his situation to a lighter side, he would see that he is still young enough to marry, he got all his life experiences out of the way so he has more of an ability to settle down, a lot of women (and men) enjoy an older and more experienced man, AND he'll be able to retire in give-or-take twenty years. This kind of thinking sets goals in mind, which means you plan on living to see your future.
Perception can keep us going just as much as a motive can. It's all about your mindset.