The amyloid-β (Aβ) proteins are responsible for amyloid plaques in Alzheimer's disease and have been the most widely studied subject in the process of fibril growth. Although much progress has been made to elucidate amyloid fibril properties at a molecular level, the full identification and characterization of all the conformational states and oligomeric structures in the aggregation process and all the conformational changes that link between those different states are still needed to be revealed. Here, we present the results of targeted molecular dynamics (TMD) simulations with explicit water to investigate the structural and mechanistic aspects of the association and the dissociation of the Aβ42 dimer. We will discuss the reversibility and the driving forces of the Aβ42 dimerization process with several order parameters along the protein aggregation pathway.