We never realized how much life we spent until we start seeing couple of grey hair in beard. We always ignored prominent grey hair on head since we are used to it from college days. But when you see grey hair on face, it sends very alarming shock wave to brain. I was almost convinced I am immortal until I saw my first couple of grey hair on face. It hits very hard since you REALIZE we are not different from rest of folks and laws of nature start affecting us as well. Also it start recollecting how much time we have "wasted" till that time and how much time we have left to do anything. Closer we look our past, more we feel uncomfortable since we don't remember many good time we had but time were we felt we could have done little better. More I think about screwed time, I feel more it affected current present, it is same like financial investment. Right decision at right time pays you in long run.
Some of minor incidents such verbal outburst in past, haunt us for long time since we hurt particular person and that causes destroying of relationship for long time. I remember those encounters especially with older generation. We never realize it until we step into their shoes.
On other side I also wonder what other "best scenario" alternative life we could have developed without wasting time what we think. I think all roads converse to same goal set by our own mind consciously or unconsciously.