We like doing and experiencing new things. People even get happy and excited just by the thought of it. Whenever we make plans for going on a vacation or travelling to new places, we get excited. Probably because the idea of travelling in itself means having new experiences. As we get to see new places, meet new people, and experience a new culture.
But, it is also true that we feel a little bit low or sad once the experience is over. You would have experienced this feeling after coming back from a vacation or a trip. People often say that you should feel lucky and you should be happy as you just came back from vacation. So, does this means that there was something wrong with your experience, or is something wrong with you that you are feeling low?
I don’t think so. I feel that we can be happy only in the present moment. So even if you have just come back from the trip, the trip and all the experiences you had lies in the past. You can feel grateful, remembering the trip which may make you happy. But your happiness will definitely take a dip because in the present moment you are not doing the same thing you did while on the trip. This doesn't necessarily mean you feel sad after having some good and novel experience. The point here is that your present happiness level is lower relative to the happiness you experienced on the trip.
Another, reason for having such a feeling can be that you have to go back to your routine life, which is something you don’t find exciting. The routine, day to day life is bad because of the fact that you are doing it again and again. Some of the components of daily life like your day job might have been exciting at first and you would have enjoyed it. But after repeated exposure to it, you would have become accustomed to it, and therefore it became boring. I believe that if we remain bored for a long period of time, we eventually become sad. That may be the reason why you may find your job very interesting at first, but that same job may become very frustrating as years go by, without anything changing with the job itself.