Question: > 
> A three line whip is an ultimatum: "Show up and vote the party line, or suffer the consequences." [source](https://politics.stackexchange.com/a/16647/25686)
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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](https://politics.stackexchange.com/a/15520) because of ethics / self control?

Yes, a Three Line Whip [requires](https://politics.stackexchange.com/a/11564/25686) 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?

Answer: 
Yes, a party could pass the equivalent of a three line whip on every single division. However most votes in the House of Commons simply aren't important enough *or likely enough to be affected* for such a hardline policy to be effective. The MPs are also members of the parties and tend to share enough ideology to vote similarly even on matters of conscience. Overall. he main effect would be to seriously annoy the MPs themselves and to mean the party whips having to constantly devise effective punishments.

On the first point, many debates have an audience of a Deputy Speaker in the chair, the MP currently speaking and those MPs who have just spoken and are about to speak. These still have a vote at the end to adjourn the session. On the other, in the last hundred years, most parliaments have had one or other major party in power with large majorities, often on the order of 100 MPs or more. In such a situation Government business will almost always pass, even if many of their MPs are elsewhere doing all kinds of useful things including talking to the people they represent.

You are also somewhat overstating the adherence to party line that US politicians maintain. The website [fivethirtyeight.com](https://projects.fivethirtyeight.com/congress-trump-score/) has given members of the Senate a "Trump Score" for the percentage of time they have voted in line with the president's position. If both caucuses were always united, you would expect one score for Republicans and another score for Democrats. Instead there is a spectrum (although the parties don't intermingle).
Remember what the consequences for defying a whip are, and what they are not. 

* The party cannot officially order a MP how to vote. That's why the generally understood convention of underlining suggestions once, twice, or three times developed.
* MPs have the **right** to leave their party. They remain MPs if they do. That is the leverage MPs hold over their party.
* MPs have **no right** to get nominated again by their party for the next election. Running as an independent will be harder. That is the leverage the party holds over the MPs.

So it is a kind of *hang together or hang separately* -- the party leadership can count on the votes of their party when it counts, and *individual* rebels will face the consequences. If whips become too common and resistance becomes widespread, the party leadership has lost control.

To give you another example, the whip is slightly stronger in Germany but the same principle applies. MdBs assemble formally into factions and faction membership helps determine committee memberships and speaking times. A lone MdB is possible, but he or she will be much less effective. MdB may decide to leave their faction (perhaps to join another) and they may be expelled if they violate the "party line" too much.