good questions. In fact, I am very glad you asked me this. So think about our three particles in each lipid:

1 is hydrophilic and 2 and 3 are hydrophobic particles. Like particles interacts strongly and will have a minima. 2-3 interaction(means 2 of one lipid and 3 of another near by lipid) should be also have an attractive term, since both are hydrophobic terms. Think about 1-2 and 1-3 interaction terms now: They should never attract each other. So they will have potentials which mimic repulsion only. No well for them and so they are monotone. Did this clarify your question?