Can priests be forced to concelebrate?
The Second Vatican Council has spoken on this question:
Sacrosanctum Concilium:
"...each priest shall always retain his right to celebrate Mass individually, though not at the same time in the same church as a concelebrated Mass, nor on Thursday of the Lord's Supper." (n. 57)
The Code of Canon Law:
Can. 902. Unless the welfare of the Christian faithful requires or suggests otherwise, priests can (possunt) concelebrate the Eucharist. They are completely free (integra tamen pro singulis libertate manente) to celebrate the Eucharist individually, however, but not while a concelebration is taking place in the same church or oratory.
The General Instruction of the Roman Ritual:
"An individual priest is, however, permitted to celebrate the Eucharist individually, though not at the same time as a concelebration is taking place in the same church or oratory." (no. 199)
"For each and every Mass is not something private, even if a priest celebrates it privately; instead, it is an act of Christ and of the Church. In offering this sacrifice, the Church learns to offer herself as a sacrifice for all and she applies the unique and infinite redemptive power of the sacrifice of the Cross to the salvation of the whole world. For every Mass that is celebrated is being offered not just for the salvation of certain people, but also for the salvation of the whole world. The conclusion from this is that even though active participation by many faithful is of its very nature particularly fitting when Mass is celebrated, still there is no reason to criticize but rather only to approve a Mass that a priest celebrates privately for a good reason in accordance with the regulations and legitimate traditions of the Church, even when only a server to make the responses is present. For such a Mass brings a rich and abundant treasure of special graces to help the priest himself, the faithful, the whole Church and the whole world toward salvation—and this same abundance of graces is not gained through mere reception of Holy Communion." (Mysterium Fidei, Sept 3rd, 1965, n. 32)
The sacred and holy Synod would fain indeed that, at each mass, the faithful who are present should communicate, not only in spiritual desire, but also by the sacramental participation of the Eucharist, that thereby a more abundant fruit might be derived to them from this most holy sacrifice: but not therefore, if this be not always done, does It condemn, as private and unlawful, but approves of and therefore commends, those masses in which the priest alone communicates sacramentally; since those masses also ought to be considered as truly common; partly because the people communicate spiritually thereat; partly also because they are celebrated by a public minister of the Church, not for himself only, but for all the faithful, who belong to the body of Christ. (Sess. 22, chp. 6)