Of course an irrevocable commitment does not per se and in itself produce happiness. But without it you do not make the total investment of self in the other that is necessary for happiness. It's all about venturing and risking everything. Staking your whole self and your future happiness on this relationship, with no possibility of another while you or your beloved are alive. High risk, high return. Low risk, low return.
Your 'return' on the relationship is really a function of your 'investment'. Partial or temporary investment produces partial 'return'. It's an inexorable cosmic law. Which is a reason why those who don't make the total investment sooner or later drift into another relationship, and so stumble into a string of serial 'relationships', and each one less able to produce the deep happiness we hunger for, eventually leading to a tragic resignation in the mistaken belief that lasting happiness is not possible. Never realising that the problem lies inside them and the quality of their commitment... What a tragic waste.
You only have to look at the sorry state of relationships to see the wreckage of low-risk, low-investment pseudo-commitment. So many unhappy and lonely people, so many hurting kids growing up into broken adults, it's heart-breaking.