Using reason to establish a worldview, life-orientation, ‘religion’ or philosophy is like building a house with a wrecking ball. Or perhaps that’s too violent a metaphor. The point is reason is not a constructive tool but rather deconstructive. Reason does not construct, build or supply the thing itself: belief, idea, value, etc. Reason only deconstructs what is already constructed, built or supplied by another source. So then, using and trusting reason alone, you will not ‘get’ anywhere. More likely, you will critique and dismiss all views until you ‘get’ to the absence of a view, which is by definition agnosticism. Reason is very popular.
Assumptions, on the other hand, are not popular. When you ‘assume’, we chide, you make an ‘ass’ out of ‘u’ and ‘me’. Assumptions, however, should not be assumed to be all bad. They are not created equal. And actually, assumptions, which none of us can avoid (and I note that it is a particularly strong assumption that assumptions should be avoided!), are the sort of things that we can (and do) actually ‘build’ with. Assumptions are thus incredibly useful and impossibly unavoidable.
If reason is a wrecking ball, demolishing every constructed system of thought we could build, then assumptions are the ground that we always build upon.