How can we reason with Jehovah's Witnesses regarding the practice of disfellowshipping fellow believers?
Well, when you look up Disfellowship in the NWT index, you are referenced to EXPEL, EXPELLED.
When you look up these two words, you get the following scriptures:
John 16:2 Men will expel YOU from the synagogue. In fact, the hour is coming when everyone that kills YOU will imagine he has rendered a sacred service to God.
John 9:22 His parents said these things because they were in fear of the Jews, for the Jews had already come to an agreement that, if anyone confessed him as Christ, he should get expelled from the synagogue.
John 12:42 All the same, many even of the rulers actually put faith in him, but because of the Pharisees they would not confess [him], in order not to be expelled from the synagogue;
Did you note that those who are doing the disfellowshipping in each case are those who DO NOT follow Christ.
If we use the Watchtower's "Birthday" logic as found in the Reasoning Book...
Everything that is in the Bible is there for a reason. (2 Tim. 3:16, 17) Jehovah’s Witnesses take note that God’s Word reports unfavorably about birthday celebrations and so shun these. -rs, p.68, par.4
...then, should we not also reason that God's Word also reports unfavorably about disfellowshipping/expelling fellow believers, and so shun the practice? If we practice expelling people out of the congregation, how are we any different than the Pharisees who did the same? Were the Pharisees favored by God for expelling people out of the synagogue? Did the Pharisees show Christian love when they disfellowshipped others?