Why time seems to flow at different speeds.

You could think of it as your mind having two gears - high and low. In low gear, you run on auto-pilot, do things automatically without thinking, exert little effort, and are generally more relaxed. In high gear, you are actively focused, embrace some stress and effort, and are generally working harder.

Time feels as though it moves more quickly in low gear (which we engage more readily while having fun - relaxing with a familiar game or tv show). We're not paying as much attention, we are not as focused. When we "tune in" time has slipped by without us noticing as much, and we experience this as time moving faster.

Time feels slower in high gear (which we engage while learning new tasks, or doing hard things that require effort like writing papers). We're really engaged in what we're doing and can readily remember all the hard work we've been through. The amount of "stuff" we've done over time as we reflect on it seems like more because we were "tuned in" for a lot of troublesome details. Time has been taxing and we experience this as it moving slower.

Hope this made sense! (somewhat weak) source: I do a healthy amount of recreational reading on psychology.

Note: you can be tuned out while doing work ("I have some mindless work to do at my job and it makes the time pass faster.") Naturally, there are exceptions to everything.