A fascinating question. I asked my friends to look at this and they came up with a couple of definitions which might dictate the way I try to answer this question.

Your question seems to be directed at intermolecular forces between similar particles/molecules 

In a past question I talked about the way water molecules are attracted to each other by things called ‘hydrogen bonds’. This aids the formation of liquid water at very high temperature and low pressures. The molecules ‘like’ being close together. Another molecule is that of carbon dioxide – these are not attracted to each other so strongly. To attract each other they need to be ‘pushed’ much closer together, only then will they form a liquid. For some molecules like butane you need really high pushes because the attraction between the molecules is so small. For some other molecules there is no force of attraction, this can almost be described as repulsion. The gases, Helium, Nitrogen and Hydrogen  are only liquefied under immense pressure (push) and low temperature (low temperature slow the individual molecules down) conditions.