There is an attraction between molecules in liquids and this is why a liquid stays together. The intermolecular distance in liquid is such that it is attracted to all the surrounding molecules. This attraction causes negative potential energy for the molecule which further depends on the number and distribution of molecules around the chosen one. This is because to collect and disperse such molecules far away from each other in order to evaporate, the heat of evaporation required is quite large. For water, it is 40 kJ/mol.