I've read a lot of anologies about voltage and how is a type of "water pressure" but i can't understand why when 2 or more resistors are in series there is a voltage drop but when the 2 resistors are in parallel the voltage keeps the same.
We think in ideals.
In the ideal circuit shown below, when you generate a voltage, with say a battery, then the applied voltage across R1 is the battery voltage. Further, your implication that there is no voltage drop across the resistors is incorrect. There is a voltage drop, it is equal to the entire applied voltage.
As you can see, attaching a second resistor will not change that voltage.
However, in reality nothing is ideal. In the real world the battery has some resistance as do the wires between the battery and the resistors as shown below.
In a good circuit R_Source will be very low but is finite. As such there is some voltage drop across it. Now when you add R2, the total current through R_Source increases and the voltage across AB will indeed be lower.
The fundamental reason is KVL. The voltage around a loop must sum to zero.
You have a voltage source producing a fixed voltage. All the other branches in any loop containing this source must sum to this voltage (but going in the other direction along the loop).
If you can make a loop containing just one device and the source, then the voltage across that device must be equal to the source voltage, even if there are other loops that can be made using the source. This is why putting more devices in parallel doesn't change the voltage across the first device.
If you make a loop containing the source and several other devices, the voltages across the other devices must sum to equal the source voltage. This is why adding another device in series will generally reduce the voltage across the ones that were there before.