This is an interesting question but there are so many different people with so many different ways of living that it becomes complicated. Some people DO live life as a sort of unconscious non rational choice, in fact some people who tend to follow feeling absent thought are choosing irrationally all the time. The whole point of a rational morality is to shift the what is guiding those choices from whim wish and feeling towards focused rational awareness and deliberation.
So I have to say that in reality a great many people choose life, choose to seek flourishing, but it is not really a conscious choice. Others choose self destructive behaviours, self sabotage, and quite fundamentally they have chosen oblivion even if only a slow and sad journey lasting the rest of their natural lives...
Others are fully aware of their own well being, and that they themselves are the primary actor causally responsible for it, and consciously choose life.
Now for these people what could be the reasons to choose life? Is the choice rational?
What constitutes a reason to do anything? A goal. Although there may be reasons for choosing certain actions to achieve particular goal, namely, that the facts of reality are such that only certain actions lead to the goal, that goal does not serve as its own reason... the goals is only the reason for those particular actions. But soon one gets into what seems like an infinite regress... chasing goals which further other goals related to other goals etc. all across the face of the universe... but they all lead back to a fundamental undeniable truth of the first person experience ... existence or nonexistence.
Bereft of humanity in a frenzy of academic torpor, a rationalist might try to view this question from the point of view of a blind universe or a platonic nirvana, which is not any kind of a point of view at all... and say there is no possible reason to choose life because there is no intrinsic goal in the universe...It must be an arbitrary choice.
To a frenzied academic attempting to see with no vision, perhaps it would appear so. But to a living creature faced with the alternative of existence and non existence, every reason, every thing, every experience, indeed the whole universe, all of it, constitutes the reason(s) to live, and the alternative is nothing which cannot be a reason for anything.
In a sense the choice is an arbitrary non-rational choice for which there are no intrinsic reasons but in another sense it is the least arbitrary and most rational choice for which there is literally every possible personal reason to make.