Sub for sub

You subscribe to me, I subscribe to you, and both counters move. YouTube classifies this as incentivization spam and bans the practice outright, which is the first reason to leave it alone.

The second reason holds even if nobody catches you. Subscribers collected this way never watch anything. For most channels the large majority of views already come from people who are not subscribed, so a padded counter buys no reach and leaves you reading a growth figure that describes nothing real. Anyone assessing the channel later compares subscribers against views per video, and the gap between the two is the first thing they see.