The self-assembly of peptides into ordered supramolecular structures, such as fibrils and crystals, is of relevance in such diverse areas as molecular medicine and materials science. However, little information is available about the fundamental thermodynamic driving forces of these types of self-assembly processes. Here, we investigate in detail the thermodynamics of assembly of diphenylalanine (FF). This dipeptide forms the central motif of the Aβ peptides, which are associated with Alzheimer's disease through their presence in amyloid plaques in the nervous systems of affected individuals. We identify the molecular origins of the self-assembly of FF in aqueous solution, and we evaluate these findings in the context of the aggregation free energies of longer peptides that are able to form amyloid fibrils. We find that the thermodynamics of FF assembly displays the typical characteristics of hydrophobic desolvation processes, and detailed analysis of the temperature dependence of the kinetics of assembly within the framework of crystallization theories reveals that the transition state from solution to crystalline aggregates is enthalpically unfavorable and entropically favorable, qualitatively similar to what has been found for longer sequences. This quantitative comparison of aggregating peptides of very different lengths is the basis of an in-depth understanding of the relationship between sequence and assembly behavior.