1) Volume
2) Gain
3) Close proximity between your sound source(geetar) and amp to cause the sound signal travelling in a loop between output(from geetar) and input(amp)
The gain in 2) is just to make things easier. Even without it, feedback can occur through close distance and loudness(volume)
The idea of using pedal to get feedback is not so much being the only way to cause feedback, but to make it easier and with a different shade of characteristic of the feedback.