The significance of everything and anything is subjective to the individual. When meaning exists for some, the same thing can be a matter of indifference for others. This also signifies that a lot of what we think is important, is purely because of how we have interpreted the event or the object or the situation. Perhaps even the feeling of sentimentality is entirely fictional, that meaning doesn't really exist. No other creature celebrates Christmas or birthdays, for them it is just another day. They don't feel disappointment, or expectation, or experience any anti-climax.
It is not to say that sentimentality is bad, or the downfall of the human race. No, it is what can primarily define us as a species. And possibly we fill meaning into every single tiny moment and thing of our lives to cover that hole we inevitably feel when curiosity gets the better of us and our answers will not suffice. It may be the fear of the unknown phenomenon of life after death, or the meaning of life, or the reason behind our existence that causes us to fill in the gaps as a peace of mind.
What we cannot do, but what we continue with daily, is let the sentimentality define our lives in the way we define our sentimentality. For that allows your day to be 'ruined', for disappointment to creep in, and for further misunderstanding. Instead, if you allow yourself to enjoy the meaning and feeling you create for everything with the acknowledgment that it may not really matter then you understand your own subjectivity. If you accept the 'bad' outcomes of the day you felt meant a lot as a part of the meaning then disappointment does not exist. It is all okay, the feeling still has the same effect.
That is near impossible to achieve I know, but it's a thought to have at least.