Handwavey explaination: the particles pair up because of vibrations in the crystal. It's modeled like a bunch of metal balls on with springs between them and you can imagine tapping one end and sending a wave of vibrations through. However, these springs are a bit non-linear and so I imagine that if you pack the atoms closer together then you will change the spring constant.
The other knob you can use to change the vibrations is the mass of the balls. This can be done by using different isotopes of the same element and the critical temperature goes down with mass.
The other knob you can use to change the vibrations is the mass of the balls. This can be done by using different isotopes of the same element and the critical temperature goes down with mass.