I believe most of us have an ideal weight for ourselves. One that we felt really good at and liked the way we looked in our clothes. A new year also symbolizes a "fresh start". Smokers decide they are going to quit, single people make more of an effort to get out and date, Unhappily married people or those in toxic relationships also strongly consider making a change.
We don't just see January 1st as being the start of a "new year" we also see it as being an "opportunity" to start a (new life). As human beings we need to have something to "look forward to" or to strive for in order to get through today. One man's opinion! :-)