Intutive understanding of internal energy
For an arbitrary fluid (not necessarily an ideal gas), a combination of the first- and second laws of thermodynamics tells us that, $$\left(\frac{\partial u}{\partial v}\right)_T=-\left[P-T\left(\frac{\partial P}{\partial T}\right)_v\right]$$where U is the molar internal energy and v is the molar volume.  For a fluid which obeys the ideal gas law Pv=RT, the right hand side of this equation is zero.  So the internal energy of a substance which obeys the ideal gas law is independent of volume.  An ideal gas is a substance which obeys the ideal gas law because the interactive forces between its molecules are negligible.  So, yes, it is because ideal gas molecules do not have attractive forces.