Most wallets use deterministic keychains and mask the generation of new addresses from the user making it easy and safe to maintain privacy.
Address reuse harms the privacy of not only yourself, but also others - including many not related to the transaction.
Every time a re-used address's private key signs a fresh transaction, the receiving party can use the history of that address to discover information about that address and its owner.
The relationship graph in a re-used address is powerfully-linked in that all of the inputs to that address are necessarily joined (via the spending authority of the private key) to all of its outputs.
This content is based on content sourced from https://en.bitcoin.it/wiki/Address_reuse under Creative Commons Attribution 3.0. Although it may have been extensively revised and updated we acknowledge the original authors.