According to relativity, there's no real distinction between space and time - yet we definitely experience them different in our day to day lives.
We are not free to roam around the time dimension in the same way we do with the spatial ones, instead we're forced to move through it at a rate of roughly one second per second.
The thing is, there's nothing in the laws of physics that suggest it has to be this way. The laws don't differentiate between the past and the future but, if this is the case, why can't we remember the future?
The prevailing theory in this field is that it's because of entropy - or the probability of a system to progress towards disorder. It's the same reason why you can run a piano through a woodchipper and get wood chips, but you can't run it backwards and get a piano.
Maybe once we crack this, we'll know once and for all whether time travel is possible, but for now it remains a mystery.