When "new faces" join us at a farbrengen we all experience a unique and expanded joy. We may draw an analogy from the halachic ruling in the case of the Sheva Berachos -- the seven benedictions which are recited (after Grace) through the festive post-wedding week. The rule is that when a "new face" is present the seven benedictions are recited, for human nature is such that "new faces" bring new joy.
Similarly, when two friends meet after a long separation their reunion evokes great happiness:

When one meets his beloved friend and rejoices and draws satisfaction from seeing him, if he has not seen him for 30 days he shall recite the blessing "Shehecheyanu" ["who has granted us life"]. (Alter Rebbe’s Siddur, Order of Blessings 12:11)
We must deduce that the joy in this case is so great that it is revealed, for Shehecheyanu is only said when we see and feel the joy. So, too, in our case, meeting with fellow Jews whom we have not seen for some time, "new faces," increases our rejoicing manifold.
The joy is compounded because we can openly and clearly show and express the emotions of unity that we feel.
This revealed unity is an important point -- for inherently and intrinsically Jews are always united. Essentially they are one because of their souls, more so because of Torah, and especially through mitzvos which are common to all Jews. A 13 year old boy puts on Tefillin just as Moshe Rabbeinu did, a three year old girl lights a Shabbos candle just like the candle kindled by the Matriarch Rivkah at the age of three, and the Matriarch Sarah, before her -- who’s candle burned from Friday to Friday. This spiritual unity always exists.
When Jews meet in one place and their unity is revealed and expressed for all to see, then they experience greater joy.
This is all the more powerful now when their gathering has a unifying spiritual theme. For they have come together in connection with the holiday of the 15th of Av and the ensuing Shabbos and with a religious commitment to increase Torah and tzedakah. In this case the spiritual and material unity is enhanced manifold.

source: Essays: Navigating the Sea of Life