It seems that Unitarianism and Universalism often go hand in hand. Is there a theological and/or philosophical reason for this? I don’t really see how they relate, but it seems that they so often go together that there must be a relationship I don’t see.
Thanks!
James
Someone who comes to find the arguments in favor of one important topic may start looking around and find the arguments in favor of another topic to be (to them) equally problematic. There may be an anti-ecclesial sentiment joining the two as well: the church is corrupt and oppressive and must be gotten out of.
Otherwise, I have yet to see any positive doctrinal connection between them, where one solidly implies the other. (Unitarian arguments of universalism from Christology tend to be pretty similar to trinitarian arguments from Christology, in that Christ recapitulates humanity and/or becomes the federal head of all humanity and/or reconciles all sinners to God by virtue of His sacrifice etc.)
Several months ago I posted a thread asking members to post arguments for universalism from specific doctrinal precepts of unitarian theology (in parallel with my derivation of universalism from trinitarian precepts, or as a few of our members derive universalism from modalistic precepts.) I don’t recall where that thread is now, but possibly someone answered and I didn’t get around to reading it. 
This would be a good thread for our U-tarian members to take a stab at it, though! 
I’ll be curious to see any results.
“Universalists believe that God is too good to damn people, and the Unitarians believe that people are too good to be damned by God.”
~Thomas Starr King
Even from near the beginning it seems (though I could do with reading the history on it more thoroughly), unitarians were emphasizing the faith OF Christ over faith IN Christ and began to see faith as a much more transcendental thing which could thus be seen more universally than within a narrow dogma. This is what led to transcendentalism and writers like Ralph Waldo Emerson:
I guess the general connection is a level of more benevolent relations between mankind and God. But what I’m referring to as regards to unitarianism is more or less a sense of inclusivism; while unitarianism neither strictly necessitates that, nor is it required for it, since unitarianism is defined as believing that only the Father is God. But it does seem to lend to it, at least insofar as the movement headed in that direction. A trinitarian universalist can definitely have inclusivist leanings of some sort, such as I do; and I hold to Emerson’s concept of the Oversoul as well.