Abstract: A theoretical framework is developed to describe the evolution of modal frequencies and damping ratios in vehicle-bridge interaction (VBI) systems. Analytical solutions are derived and validated against finite element results, with the effects of the vehicle-bridge frequency ratio, mass ratio, damping ratio, and relative vehicle position systematically examined, along with the governing mechanisms near resonance. Key findings include: the frequency and damping ratio exhibit markedly different variation patterns near resonance; for a simply supported beam, the bridge frequency attains its extreme value at midspan, governed primarily by the vehicle-bridge frequency ratio; when the mass ratio is much less than unity and the damping ratio exceeds unity, the bridge damping ratio reaches its maximum at midspan, indicating a vehicle-induced damping enhancement effect; as the mass ratio increases, this maximum damping ratio can be reduced to the minimum, with the maximum values shifting symmetrically to both sides of the bridge.