The apostle Paul was a great missionary, sometimes traveling with companions and being imprisoned for their faith (Acts 16:25), and sometimes isolated and alone (2 Tim 4:16-18). Paul said we were all engaged in the ministry of reconciliation (2 Cor 5:11-21).
Jesus himself often needed to withdraw in seclusion (Lk 5:16) and sometimes became frustrated from his followers dim eyes and heart, and their failure to understand (Mt 17:17). Jesus gave his followers the ability to, and ministry of, fishing for men (Mrk 1:16-20).
Being a follow of Jesus can be lonely. Being in the paid full-time ministry can be even more so. There are so many great benefits to being the in the ministry (referring to as a paid supported vocation - c.f. 1 Tim 5:17-18; 1 Cor 9:7-14) and anyone who does it for very long had better love it immensely, for it difficult and often quite lonely. Part counselor, part teacher, part herald, part leader, part comforter, part preacher, part trainer, part visionary, and often in our fellowship; part administrator, planner, organizer, and mascot. All the while, there are typically not many (or any) others near by that can relate very well. People of course mean well, and relate in some ways, but the ministry is such a specific calling and vocation type that it can be hard to completely understand. I imagine it to be a cross between the schedule of a doctor who is always on call, the emotional toll of a professional psychological counselor, having the fickle fanfare of the latest winning (or losing) team, all the while being supported financially by people to do and say things that at many times they don't want to hear.
A word to those who are, or have, considered ministry as a full-time vocation; allow God to purify your heart and motives, for it is truly a calling. Something that you must desire to do above all other things (vocationally), and something you are willing to endure the times of lonliness for.
I doubt I could know something as glorious, and as difficult as following Christ. And to be able to be involved in ministry full-time is such an amazing gift and joy, but it is not without its trials and difficulties, chief of which perhaps is loneliness.