From the diaries of Rabbi Leibel Groner:
8 sivan 5729
Yechidus Night, until 3:30
Someone related that he asked the Rebbe tonight in yechidus why one must Daven with a Minyon – seemingly one’s Davening is then limited to the environmental constraints of the larger Tzibur, when one Davens in seclusion, however, one can concentrate and contemplate etc. [according to his abilities, without such constraints].
The Rebbe responded: A Yid is a Mashpia [influencer] and a Mushpa [influenced from others], when one Davens with a Minyon he receives from the other Yidden with whom he is Devening, however, when Davening alone he has only as much as he can give himself…
About secluding oneself, the Rebbe said: To seclude oneself alone, in a room, is not a virtue. To the contrary, we find that a most serious punishment is ‘solitary confinement’. A virtue is to be with others – and there to be in seclusion [with one’s thoughts].
Someone related that in Yechidus he told the Rebbe that his wife has ‘questions’ on G-d’s conduct towards her, etc.
The Rebbe responded, that when his father-in-law, the Rebbe [Raya”tz], was in Philadelphia, a Jew walked in and asserted that because he had ‘questions’ on G-d, it causes him to lack in his fulfillment of Mitzvos, to which my father-in-law, the Rebbe, replied, “I also have questions”. The Rebbe concluded, “Tell her that I, too, have questions…”