I've lived though eighteen New Year's Eves, made eighteen perspective-altering resolutions, and let myself down eighteen times. For some reason, though, this has never bothered me. My resolutions have fallen under a wide range of intensities and levels of commitment. They're ranged from things like "keep a diary" to "learn to speak French fluently". None of these, no matter how easy, have ever been kept up with until the next New Year.
This conundrum occurs annually for a number of reasons. First of all, I can proudly say that I am a serial procrastinator. 'I'll just start tomorrow' should literally be my middle name. All of my attempted resolutions have started, (and promptly ended), around late February. While certain levels of commitment I still manage to find achievable, even telling myself to stop eating meat took a little while to take effect.
Reason number two: Things change. What you want at the end of one year may not be reasonable or even necessary at the beginning of the next one. A goal made in the spur of the moment may be too big to handle or too little to stretch over a whole year, and that's okay! You're allowed to change your mind. People change, and so do plans.
The last reason for this phenomenon is that the whole concept seems silly in retrospect. What's the point? Is it implied that if you don't keep up with your resolution, then you're a failure? Was the whole year a waste of time and effort? It just seems like a lot of pressure to put on ourselves when there's so much else going on in our lives.