Brochas: Daf Lamed Daled amud aleph

The Rabbis taught in a brysa. Three things are bad in excess, but fine in moderation. And they are yeast, salt and refusal (to become Shliach Tzibor)

The Eitz Yosef explains the connection. Yeast and salt both strengthen mixtures into which they are placed if included in small quantities, but ruin the mixture and weaken it, if added to excess. Similarly an over zealous and/or prolonged refusal to serve the community as Shliach Tzibbor causes the klal to lose the opportunity to come together as a united body in the service of G-d. And the one who demonstrates this trait demonstrates the lack of worth he attributes to taking up his communal responsibilities.