In this paper, the user association and resource allocation problem is investigated for a two-tier HetNet consisting of one macro Base Station (BS) and a number of pico BSs. The effectiveness of user association to BSs is evaluated in terms of fairness and load distribution. In particular, the problem of determining a fair user association is formulated as a bargaining game so that for the Nash Bargaining Solution (NBS) abiding the fairness axioms provides an optimal and fair user association. The NBS also yields in a Pareto optimal solution and leads to a proportional fair solution in the proposed HetNet model. Additionally, we introduce a novel algorithmic solution in which a new Coalition Generation Algorithm (CGA), called SINR-based CGA, is considered in order to simplify the coalition generation phase. Our simulation results show the efficiency of the proposed user association scheme in terms of fairness and load distribution among BSs and users. In particular, we compare the performance of the proposed solution with that of the throughput-oriented scheme in terms of the max-sum-rate scheme and show that the proposed solution yields comparable average data rates and overall sum rate.