So by definition, we said solution formation is a formation of intermolecular bonds between X and Y. Well, before a bond is formed between X and Y, the bonds between the XS and the YS must break. So before X and Y forms a solution, intermolecular bonds between compound X and intermolecular bonds between compound Y must break. Once all these intermocular bonds break, then intermolecular bonds between compounds X and Y must form. Now remember, the breaking of a bond is endothermic. So these guys are endothermic.