First off, I hate when people look back on a week and say, "Man that week flew by" or, "Ugh! This week will never end, it's going so slowly!" Unless the earth slowed down on its trek around the sun, the days and weeks still went by at the same rate. And yes I am somewhat aware of relativity, but as long as none of us were going light-speed, I believe we were all going about the same speed as we stayed attached to the earth. (That being said... I can't help but say it sometimes)
I'm going to present two ideas, none of which are mine, but that do seem to provide likely answers as to why we get these feelings of time 'flying' and why we always have to let everyone else know that it is for us.
1.
I read this one awhile ago, but I can't recall where I read it. Its simple, yet makes complete sense. After one year of life, that past year was 1/1, or 100%, of your experience. After your twentieth year of life (of which I am about 4 months from completing!!!), the previous year was 1/20th of your experience. So each year, since we are unable to expand our brains, causes the percentages of our brain to get reconfigured. While this does seem too simple, and it probably is, it does make sense and is most likely correct to an extent.
2.
This one comes from Jeff Hawkins (creator of the PalmPilot) in his book, On Intelligence. One of the main subcategories to his overarching theory is that the brain's neocortex has a hierarchy of cells. Thus, there are lower levels and higher levels to it. The lower level take in basic sensory experience and then relays this up to the next level. At the next level, those cell's input is comprised of the various neuron firings from the lowest level. As this level recognizes patterns, it relays to a higher level where they see a pattern and begin to do different things with it. This a very poor reconstruction of his argument but I think it will do for this.
Now when these patterns are something we've seen before, they follow a certain path and this pattern fits the previous path and so we recognize it (from memory). When it is new, it does not follow a common pattern. So while we see a person and we recognize (from memory) that it is a person on one of the mid-levels of the hierarchy (because all our low-level inputs show a pattern consistent with the inputs a person has normally given us), that is where it stops and we cannot associate a name with the person (which would be a higher-level task). So when a new series of inputs fits a pattern we've already seen, it is 'recognized'. If an experience is completely new, our neocortex takes notice of it and it goes all the way up the chain to the Hippocampus, where it is stored (in a sense).
Now how does this relate to why time flies???? Well Hawkins suggests that over time, and after more and more experience, we see less and less 'new' things. So while we may see 500 baseball games, they start to blend together because not a lot of 'new' experiences are occurring. We remember our 'first baseball game at Wrigely,' but we most likely won't remember the 73rd one, unless it happens to be the game the Cubs win the World Series (which would certainly be 'new' for us!!). In his diagram then, as a pattern is familiar, it will follow similar paths that we already have. If nothing new results, then it doesn't make it to the hippocampus. But if a guy spills his beer all over you in the 8th inning, that is most likely a new experience and it makes it all the way to the hippocampus to be stored. So while you may not be able to tell a friend how many strikeouts Zambrano had, you will certainly remember that spilled beer.
Again, I seem to avoid the question! But here is what it comes down to. As we experience more and more, less and less becomes 'new', because we have all this background that our brain is able to relate to the current situation. I think Hawkins uses the quote, "The more you experience, the less you remember." So in regards to the passing weeks of our life, as we experience less and less new information, it tends to not make it all the way to the hippocampus and as a result, we do not remember it. So weeks where we do 'the same old thing' are not the 'memorable' ones and our brain aptly 'discards' them.
another good example is people's names. Lets say you meet a new person and their name is generic, like Sarah. If you know 50 Sarah's that is most likely going to be tough to remember. But if her name turns out to be Jerra, it will most likely be foreign to you and you'll comment "Wow, that is a different name!" and you'll most likely remember that person and their name. While this is something we've all experienced, I'm still amazed at how Hawkin's theory is able to explain it scientifically and anatomically.
Side note: On Intelligence has become one of my top 2 or 3 books of all time. I've read it in two days and am disappointed it is over with. At the beginning of the book Hawkins says that when people read his book, they find themselves nodding in agreement with the things he says and how they can relate it to their lives. What is brilliant about his book is that it takes all these different aspects of our lives (on the scientific as well as social levels) and provides a theory of the brain that explains almost every part of them! While reading it, I did just what he said people usually do and thought back to all these instances that have occurred in my past that this theory explains. Now, after reading it through, I experience something and immediately think back to his book and how it perfectly explains what just happened (like with why people say "time flies").
I strongly encourage everyone to read this book! I know people always bombard you with these different books that they think is the greatest book ever, but seriously, this will change how you view your every-day life. I'm not saying you're going to quit your job and divorce your wife, but it will provide insight into why we do things we do and why we experience different things the way they do.