Role of Al in aluminosilicate glasses remains misunderstood; at low concentration it increases the glass resistance to hydrolysis whereas at high concentration an opposite effect is observed. To understand the origin of the phenomenon at atomic level, we performed 577 MD simulations through potential mean force (PMF) method to estimate the activation barriers and to correlate statistically with their corresponding structural features of silicate and aluminosilicate glasses. PMF simulation results validated by the experimental studies revealed that Al is very easy to dissociate, but increases the resistance of glass towards water through significantly increasing both the strength of Si and network connectivity. In contrast, at high Al concentration, preferential dissolution of Al weakens the silicate network, and so the glass resistance becomes very poor. Activation barrier for dissociating bonds around Al is 0.46eV, which is significantly less than Si in pure silicate (1.22eV) and Si in aluminosilicate glass (1.34eV). Si with Al as second neighbour had significantly higher activation energy than Si in pure silicate glass.