Over the past week, I've seen a couple news stories about Russian aircraft flying unusually near US military personnel, including warships and US aircraft. What does Russia gain from this, and why are they doing it now (as opposed to, say, three years ago)?
They're trying to show off, and show to the rest of the world that they are powerful etc. The reasons for why they are doing this are becuase of disagreements the US and western Europe currently have with them. These are: Syria, Ukraine, and Cold War 2.0.
Syria: The US and their coalition and Russia aren't happy with each other because they support different groups fighting in the region.
Ukraine: Becuase NATO and the US claim that Russia has annexed crimea, while Russia denies that. Also the shooting down of MH 17 is a point of conflict between the two sides.
Cold War 2.0: NATO has increased military exercises and troop numbers along the NATO border with Russia, in countries like Latvia, Lithuania and Estonia.