Not exactly automatically.

Seeding is opt-out, not opt-in… but it is usually a default that has to actively manually overridden. Most users never touch those settings. The average pirate downloading a torrent is seeding whether they know it or not.

The protocol absolutely does not enforce seeding. A client can lie to the tracker, cap upload to 0k. BitTorrent has no mechanism to compel one to share. Leeching a file, downloading and sharing no forward packets is possible. While the "social contract" of seeding is entirely a norm enforced by private trackers and community shame. It is not the protocol itself.