Sri Lanka is home to multi-cultural communities. It is the responsibility of the people across various religions, and communities to develop and maintain harmony with each other. Historically, the Sri Lankan Muslims and Sinhala Buddhists had an excellent relationship. Recently, the ethnic harmony between these two communities has been strained reflecting the fault lines running in a current social structure which lead to ethnic tensions, social animosities, restlessness, and disharmony among communities, amidst diverging political ideologies. Hence, this study focuses on identifying the root causes that wreck the harmony and social stability of the country. Hundred and fifty students from the South Eastern University of Sri Lanka were randomly selected to respond for a structured questionnaire, and fifteen formal interviews with students were also conducted to validate the questionnaire data. The secondary data were collected from various sources of information. The collected data were analyzed using descriptive and basic statistical analytic techniques, and findings of the study were presented in the form of table and text. This study underlines the array of reasons, and root causes that prevent the harmony among Sinhala Buddhists and Muslim communities, such as ethnic differences, spreading hatred via social media, extremism that uses religion to forward their extremist ideologies. This study concludes with the argument that the government and people who strive for social harmony should act with commitment and dedication in the efforts to build harmony among religious communities in post-war Sri Lanka.