Humans like to solve problems, answer questions, figure things out, understand the "why?" of things. One can say that there is a sort of agitation or tension when something is presented, and we can't figure it out. To quell that agitation or tension, we seek answers. But the answers don't necessarily have to be true -- only enough to satisfy the desire to have closure to our questions.
One method of transforming an unknown into a known is to anthropomorphize the unknown. Some cultures answered the question, for example, of what thunder and lightning were by saying they were the actions of a being which existed in the atmosphere somewhere. That satisfied the question of what the thunder and lightning were, but also started a journey of questions about the being which caused the thunder and lightning. Stories developed over time about this being, to answer those questions -- to satisfy our desire to know. Whether or not the explanations were accurate is irrelevant -- what mattered was that they were believable. Without other explanations available, they survived without competition. But as each was developed independently by different cultures, they varied. Thus began the religions of the world.
These cultural explanations were passed down via parents to their children, who are psychologically more amenable to trusting and accepting fully the ideas of adults around them. This is a side-effect of an evolutionary adaptation benefiting the children, as those who didn't adhere to their parents' warnings about natural dangers were more prone to succumbing to them. Without examining which admonishments and lessons were beneficial individually to their own survival, children grew up accepting them as a whole package, and passed them on to their own offspring. "If it ain't broke, don't fix it."
But if it wasn't for the small faction of enterprising independent thinkers who either stumbled upon or actively sought out other explanations or knowledge, cultural innovation and technology would not exist. There have always been some who are left scratching their heads and furrowing their brows at beliefs which are accepted by their peers. Sometimes their explorations into alternative solutions are met with failure, sometimes with success, but their basic desire for answering questions is the same as that found among basically all of us. It's just that for whatever reason, they aren't as amenable to accepting the answers offered to them at the time, and seek their own.
And thus begins the conflict. Those who have accepted the cultural answers to the nagging questions have found their peace, regardless of the accuracy of their beliefs. Those who did not accept those cultural answers still have that nagging pang to investigate further, and if those investigations uncover the inaccuracy of the culturally accepted answers, then the former group is thrown into the tension of "not knowing" because the accuracy of their beliefs is now in doubt.
And once again, seeking to quell that tension, they have two options -- to change the answers they believe, or to rally against the competing ideas. Sometimes, the alternative explanations are not as easily understood, or they are incomplete, or simply the fact that they invalidate the basis of an accepted belief makes them threatening. An alternative explanation might provide a more accurate answer for one question, but accepting that answer leaves many more as yet unanswered, and the agitation of all those things unanswered is too frightening to accept.
You'll see this often when theists and atheists debate. The theist will say "Oh, so you think this is how it happened? Well, then how did that happen, and how do you explain this, and what about that?" The atheist will respond "Well, I don't know for certain" or "Possibly it happened this way" but these answers are unacceptable to the theist because they leave too many agitating, uncomfortable unknowns. They still don't understand the answers to the questions they ask of the atheist, but are satisfied with ascribing them to the actions of a being -- the classic god-of-the-gaps response.
For "spiritual fulfillment" one often sees how a god is employed to fill other gaps. The idea that one is never truly alone during times of stress because a god is there with them is a comforting thought, accurate or not, and relieves the tension and agitation of those situations. If a loved one is injured or sick, the belief that "all is in god's hands" calms the tension and agitation of not knowing the outcome of the situation. When atheists debate these ideas, they are removing that relief of tension and agitation but provide no alternate replacement, and thus elicit defensive responses from the theists.
Atheists and theists both start from the same point -- seeking a source of answers to uncomfortable unknowns. Theists found a package which satisfies all regardless of accuracy or evidence, while atheists maintain tension and agitation over inaccuracy or lack of evidence. Atheists still have uncomfortable unknowns, but quell that tension and agitation with the pursuit of accurate evidence-based answers.