Work engagement has been recognised as a strong indicator of work performance, while relationships in the workplace with co-workers and supervisors has been found to have a positive impact on work engagement. However, there is limited knowledge in the construction literature on how work relationships lead to work engagement. Drawing from the field of positive organisational behaviour, this paper explores how workload, sense of purpose and positive emotions could build the underlying mechanism between work relationships and engagement of project professionals. Using scales from the ASSET model, data were collected in an online questionnaire survey. A total of 183 valid responses were analysed using structural equation modelling. The results showed that work relationships are positively related to sense of purpose and negatively related to perceived workload, while workload and sense of purpose are positively associated with positive emotions and work engagement. Positive emotions are a mediator in the relationship between workload and engagement, and in the relationship between sense of purpose and engagement. This study highlights the pivotal role of workload and sense of purpose in increasing the positive emotions experienced by employees, and the concomitant experience of greater work engagement with the organisation.