why is it that when i sleep longer, i tend to become sleepier..

Yeah that kind of is weird, but I think I have figured it out. If we are used to being in a state of sleep, then it will be hard to get used to not being out of that state. I've noticed that there are times where I slept for six or seven hours and it took me only a little bit to get going. However, there are times where I slept far longer and thus it took me far longer to get up. Our brains get into this comfortable state. We continue to sleep, relaxed, ease into a world where there are no worries. The pain of the world is something that has long since faded. Then we have to reenter the real world. It is rather disorienting and than I feel sleepy for a little bit, as do many others. I think that getting too much sleep is hard to shake off from than not getting enough sleep.

Maybe it is because the longer you sleep your body becomes weaker although in contrary it should be more rested, but then again, exercise pumps blood faster and makes the body more active so being inactive longer might trigger this sleepiness. This is just my theory though.

yes it happens when your body hasn't adjusted to that sleeping-waking up pattern yet... but if you decide to do it everyday, say, sleep and wake up at the same time, eventually your body will adapt to it and it'll feel easier for you.