Initially political parties form out of a basis of convenience. They bring together like minded individuals as it makes delivering their ideological policies much easier whilst giving the voters clear identification to what ideology they align themselves with.
However, political parties become problematic with age. The older and larger a political party gets, the more members of different ideological opinion form. Take the modern Conservative Party and the Labour Party, both are quite old parties that have been around for over 100 years. Initially they would draw support from a narrow path, but after a while, as the grow and they get more successful, people who share 7/10 of their views would join, and so on and so fourth until the party has different wings.
Take the Conservative Party, it has two very notable wings, The New Right (Thatcherism) and the "Modernisers" which David Cameron is the champion of. However, even elements of the Thatcherite wing of the party have splits, some are more libertarian, and others are not.
The Labour Party is similar, it has two distinct wings. The old socialist wing and the Blairite Wing (New Labour). Both wings strongly dislike one another, but they reside under the same party which at an election can give voter a false impression to what their ideological stand point is.
The problem comes with the move to center. Center right voters make up about 35%, center left make up 35%, the majority however make up about maybe 20% of the core voters and these are either left of center or right of center, this is the "swing" voter (the other 10% includes the far right and far left). Parties aim to attract swing voters as they traditionally hold the balance of power. Parties therefore, Labour and Conservative, move towards the center and their policies quickly align with one another, making them very similar parties with only minor differences.
In the end, the political system in a First Past the Post electoral system moves towards the center and forgets about everyone else as politics becomes not about enacting ideological policies, but rather winning and securing the career of a politician.
The Party whips are another key factor in party political systems, they disincentivize parliamentarians of a party to vote with their heart, but rather with the political party.
Then when factions of the party break off and rebel, new parties on the left or right form. They champion the right of center or left of center policies. However, come the election in a First Past the Post system, if a voter votes for the new lets say right of center party as they want a more right wing MP, they will be punished by the voting system as the Conservative Party (lets say they are the current MP in constituency X) will lose votes making it easier for a left wing party to win that seat on a lower percentage of the overall vote, some MP's get in with less than 30% of the constituency vote,
MP's become reluctant to want voting change as it would damage and threaten their own position in power and the two big parties fear that. This creates what people coined as the "two party state", two political parties in power forever, both are centralist parties and both become a closed political elite.
This makes the political party system in a First Past the Post system perhaps the most anti-democratic, supposedly "democratic" form of government.