The transition of power systems toward higher shares of Inverter-Based Resources (IBRs) is reshaping the technical needs of the power system, fragmenting the naturally bundled portfolio historically provided by synchronous generators (SGs) and presenting spatial and temporal interactions between system needs and services. Understanding these couplings is important for determining how system services should be coordinated and delivered under different operating conditions. This paper presents a data-driven framework for identifying and quantifying dynamic couplings between power system needs using measured output trajectories. A windowed stochastic subspace identification method is used to obtain a dynamic representation of the system, from which trajectory sensitivities are derived to characterise interactions between active power, reactive power, frequency, and voltage across network locations. The framework is demonstrated on modified IEEE 14-bus systems representing synchronous-generator-dominated and IBR-penetrated operating conditions. Results show that the magnitude, direction, location, and duration of system couplings vary significantly with network composition. In particular, active- and reactive-power support can both influence frequency, with these interactions becoming increasingly spatially and temporally dependent under IBR penetration. The proposed framework provides a technology-agnostic basis for identifying critical service interactions and informing coordinated power system service delivery.