Driving forces for the pressure-induced aggregation of poly(N-isopropylacrylamide) (PNiPA) in water are investigated by performing extensive molecular dynamics simulations. First, we observe that the model short oligomer of PNiPA with a modified OPLS-AA force field in water shrinks with increasing pressure. At varying pressures, the potentials of mean force (PMFs) between a pair of N-isopropylpropionamide (NiPPA) molecules, the repeating unit of PNiPA, are obtained and decomposed into the nonpolar and Coulombic contributions. The nonpolar contribution is the PMF between the hypothetical nonpolar NiPPA molecules in the solvent, which is mainly due to the molecular volume effect. The attractive force between NiPPA molecules is enhanced at higher pressures in agreement with the behavior of PNiPA. This pressure dependence of the PMF is caused by the growing nonpolar contribution at higher pressures. In contrast, the Coulombic contribution to the PMF becomes higher overall, making the mean force less attractive or more repulsive, with increasing pressure. The strength of the aggregation and its pressure dependence of the nonpolar contribution in water are closely reproduced even in nonpolar solvents. The degree of the pressure dependence is explained by the isothermal compressibility or the tightness of the solvation shell around an isolated solute, without regard to the existence and variation of hydrogen bond networks in a solvent. The role of hydrogen bonds in the aggregation of NiPPA and PNiPA molecules is also discussed.