If current leads the voltage in a load, it means the load is mostly capacitive and reactive power is negative. What I don't understand is that reactive power depends on sin(phase of voltage-phase of current) so if current is leading it means I have smaller value for phase of current and sin has a positive value. So how come we alway talk about negative reactive power for current leading loads?

A load that has a lagging power factor is, by convention, said to be receiving reactive power from the source. A load that has a leading power factor is, by convention, said to be delivering reactive power to the source. In a vector representation of AC circuits, inductors are given the value +jX and capacitors are given the value -jX, so that leads to the the use of +jI for lagging reactive currents and -jI for leading reactive currents. Therefore, lagging reactive power is positive and leading reactive power is negative.

Sometimes a leading power factor is given a negative sign, but that is not a good practice, because power = V X I X pf and negative real power is power that is returned to the source by the load as a motor does during regenerative braking or a utility customer does by generating more power than they use with a renewable energy system.

Good question.

Wikipedia's article on Power Factor states: