The series voltage doubles because batteries make a potential difference between their terminals. So let's assume 2 AA batteries with a nominal 1.5V difference between their terminals. One battery itself makes a difference of 1.5V. Connecting another in series, it adds another 1.5V difference, relative to the 1.5V difference the other has already made.
To say two identical batteries in parallel doubles the current is a little misleading, because the current that flows through the batteries is determined largely by the load attached to them. If they are attached to a high resistance, very little current will flow. Attached to a low resistance, a high amount of current will flow.
Taking this to the limit, what happens if we attach a battery to a zero resistance? Does an unlimited current flow? Not in practice, because the chemical reactions in the battery can only happen so fast. Electrical engineers model the underlying chemistry as an "internal resistance", as if there's an imaginary resistor in series with the battery. This puts an upper limit on the current a battery can supply.
With two batteries in parallel, you have twice the battery plate area able to react. The same thing can be accomplished with a different battery size, such as using C cells instead of AA cells.
Or as an electrical engineer would put it, the internal resistance of each battery is in parallel. That halves the effective internal resistance, and thus doubles the current that the battery could supply into a zero-resistance load.
Of course in practice batteries aren't used to power zero-resistance loads, since that would just mean a dead battery. Rather, the objective is typically to make the battery's internal resistance much lower than the resistance of the load, such that that the battery's internal resistance is negligible.