My father was LDS. My mother was JW. My relatives were Humanistic and Reconstructionist Jews (which my parents were raised as). There is sincerity in the ranks of the LDS, but a lot of what you see in public can be and truly is just show.
The best way I can describe the smiles on the faces of the missionaries that you see are "forced sincerity."
The "excessive happiness" on the faces of missionaries "elders" is very much Part 1 of the Love Bombing that all missionaries and LDS members are trained to engage in. Non-members are meant to be "smiled and loved" and "fussed" all over, even to the point of pretending that you are interested in the non-member's religion, attending their religious services once or twice, and then asking them if they can "practice" a lesson with a newer missionary at your place. That's how it starts. (Yes, they are trained to do this like Witnesses are trained to publically preach).
Mormons "love" their non-member neighbors and workmates the same way. They feign interest in them to the point that the non-member feels obligated to go to church with them. Part 2 of the Love Bombing then occurs at the Ward church building and continues on, hoping to get the person baptized and qualified for Temple attendance.
Temple attendance and work is the main goal. Mormon religion is all about going to their Temples and baptizing for non-Mormon dead people. They love to convert Jews because they can't baptize dead Jews unless they are relatives of the converted proxy.
A lot of what you see in LDS people are their attempts to stay clean enough in daily life to remain holy enough to qualify to visit their local Temple. If they mess up in any way, they have to confess their sin to their Ward bishop. If they don't do this, they can't visit their Temple and do this most important work (even more important that missionary work). Even the most private and embarrassing thoughts and personal things must be confessed to the Ward bishop--who is a man in his 50s--and children have to confess too!
Some live this way because they want to advance to the highest levels in the Church and Temple service in order to advance in the afterlife. They are told to tell the press that Mormons don't believe that they can become gods over their own worlds if they advance enough in this life, but they do. Some of what you are seeing is this, believe it or not.
There is some fear to do right too, of course, but that is mainly among those who do not fit in with the LDS standard: i.e., LGBTQ people, women, and yes, people who are not white men.
They have become a little more normalized into the culture than the Witness, but only because they have been around longer and their end of the world prophecies that have failed have been pushed to the back for over 100 years (until just recently, that is--the latest prophet is pushing the idea that "we are very near" the end). They tend to lightly celebrate Christian and worldly holidays but mostly to purposefully fit in with the objective to try to proselytize about the "Restoration." This another part of the smiles and happiness. They are told to act like close families for this purpose.
First answer here: Been reading for a while.