  By providing mathematical estimates, this paper answers a fundamental
question -- "what leads to Stokes drift"? Although overwhelmingly understood
for water waves, Stokes drift is a generic mechanism that stems from kinematics
and occurs in any non-transverse wave in fluids. To showcase its generality, we
undertake a comparative study of the pathline equation of sound (1D) and
intermediate-depth water (2D) waves. Although we obtain a closed-form solution
$\mathbf{x}(t)$ for the specific case of linear sound waves, a more generic and
meaningful approach involves the application of asymptotic methods and
expressing variables in terms of the Lagrangian phase $\theta$. We show that
the latter reduces the 2D pathline equation of water waves to 1D. Using
asymptotic methods, we solve the respective pathline equation for sound and
water waves, and for each case, we obtain a parametric representation of
particle position $\mathbf{x}(\theta)$ and elapsed time $t(\theta)$. Such a
parametric description has allowed us to obtain second-order-accurate
expressions for the time duration, horizontal displacement, and average
horizontal velocity of a particle in the crest and trough phases. All these
quantities are of higher magnitude in the crest phase in comparison to the
trough, leading to a forward drift, i.e. Stokes drift. We also explore particle
trajectory due to second-order Stokes waves and compare it with linear waves.
While finite amplitude waves modify the estimates obtained from linear waves,
the understanding acquired from linear waves is generally found to be valid.
