Life brings to you so much goodness, having just one lifetime is not enough. But that's how it is. One life, one lifetime. Despite its short stint in this world, life brings up so much to enjoy, so much to feel happy about, so much to explore and so much to cherish - its way too much one can gather fully.
We all grow up with varied experiences. Right from our childhood to adulthood, we come across people, situations, moments & feelings that shape our life. Some of them bring immense joy while some bring along unhappy moments. While we look back at happier times with a smile and a deep sigh (in our heart!), we keep the negativity of unhappy moments and never let it go. This is exactly where our own negativity starts growing. As humans we harbor feelings for everything around us and particularly keep those which gave us pain or hurt us. And this goes on all our life. We never realize that everything that happened (whether good or bad), happened for a reason: to enrich our life and move it in a direction that life has destined for us.
Perhaps an example would clarify: 1) X person had a painful childhood with numerous sufferings and hard times...yes, it is sad to have such a phase in life and to look back is even more painful. But think of it, all that experience would have made that person so strong willed. He would perhaps fare much better under stressful situations in his adulthood. Or may be he would strive harder to achieve things he was deprived of in his childhood.
2) Person Y goes through certain bad phases time and again. He feels 'Why me?'. He feels his life has deliberately subjected him to agony and pain. But it is certainly not that way. As I said earlier in one of my post, some of us go through much harsher experiences while others don't, because some of us need to learn more (to become strong willed). At the end of those troublesome time (yes, no time lasts forever! Troubles come to an end too.), he would have learnt to deal with so many things - people/situations etc.
So if you continue the process of just living in past, nurturing your past pains/ agony, harboring even more negativity(than those moments actually intended), you would never be able to relish the happiness that life brings to you. The whirlpool of negativity will always pull you deeper into it and never let you rise up to see the sunshine of life. The best you can do is to look back, think of what you learnt from the unhappy phases, think of what you are because of those experiences (again on a positive note!), forgive people who were instrumental in those phases and let go. Once you do this for every unhappy moment you still harbor, you would be able to truly cherish the joys you have. It is certainly difficult to do this but persistence creates miracles!
With no negativity to cling onto, you then need to know that happy moments of life are extremely precious - not because happiness is precious (it is actually the most abundant, easily available commodity & 'Free' !), because every joyous moment is special. It is your very own, very unique, very special. No one ever on this earth would feel the same moment, the same way as you felt about it. No one will ever be able to know the depth of what you felt about it. It will always be there in your heart - like a precious little diamond. Enjoy it, enclose it in your heart, don't let it go. Even if something (person/object) related to that phase or moment was not yours, at least it made your heart feel warm & contented. May be something much nicer, much better awaits you in future. But those moments were yours, that feeling belongs to your heart and will always be there. You just need to cherish it with a smile. You just need to feel its goodness and be thankful about it.
Life is so unpredictable, it changes without giving a hint. Although you cannot ever be fully prepared, you can at least cherish what you have. As for the changes, you just need to know that things change for us because we humans are gifted to feel it, enjoy it, express it. Otherwise we would be better off being animals (all they can do is live quietly). So squeeze every bit of happiness in your lifetime in the best way you can - At the end of it, it will all be worth the effort!