The grid-forming (GFM) concept is an inverter control method that deploys the inverter’s power modulations to regulate the system voltage and frequency. A variety of GFM controller topologies can be found in the literature. Mainly a GFM controller consists of a layer that mimics synchronous machine characteristics and a current-limiting loop. Depending on the controller topology and parameters, a GFM inverter’s dynamics can spread over a wide bandwidth leading to a wide range of interactions. The full disclosure of the root causes of interactions that can be excited by a GFM inverter is still lacking in the literature. Therefore, in this research small-signal, model-based eigenvalue analysis is conducted on commonly-used GFM controller topologies with different ac- and dc-side system configurations to reveal the full causes of interactions that can happen in a GFM inverter system. The virtual electromechanical interaction between GFM inverters and other GFM inverters and synchronous machines, high-frequency network interactions, and interactions between the dc-side circuitry and GFM controller, LC filter components, and the governor-turbine of synchronous machines are revealed and verified by PSCAD/EMTDC simulations. This comprehensive analysis unfolds the main driving factors behind the critical interactions in GFM inverter systems and proposes effective mitigation methods.