The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultrathin films on the most commonly used substrate is a long-existing question under debate. Either strain effect or nonstoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [X. R. Wang, C. J. Li, W. M. Lü, T. R. Paudel, D. P. Leusink, M. Hoek, N. Poccia, A. Vailionis, T. Venkatesan, J. M. D. Coey, E. Y. Tsymbal, Ariando, and H. Hilgenkamp, Science 349, 716 (2015)SCIEAS0036-807510.1126/science.aaa5198], one more mechanism, namely, the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultrathin films as well as superlattices. Starting from the very precise descriptions of both and , it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for heterostructures. Our paper not only explains the long-term puzzle regarding the magnetism of ultrathin films but also sheds light on how to overcome the notorious magnetic dead layer in ultrathin manganites.