A three line whip is an ultimatum: "Show up and vote the party line, or suffer the consequences." source
What's stopping the leaders of a party from three line whipping every single vote? Is there a limit on the frequency of its use, or is the reason it's so uncommon because of ethics / self control?
Yes, a Three Line Whip requires that you are present at the time of voting and that you vote in the way your party wants. If everything gets whipped, you don't have people in parliament anymore, you have only political parties.
In the United States I'm used to having only 2, well established, political parties. Republicans always vote together. Democrats always vote together. It's an outlier when a handful of members "cross the aisle" to vote against their party. This makes the stance of the party front and center, and it very much seems that congress members first serve their party before the people of their state.
Is this an attitude change between the nations on how political parties are ran? If a political party has the power to Three Line Whip, why not do it more?