Why do people rely on their partners for their happiness/fun?

Relationships are not always laughing and partying and all fun and games, life happens and life gets into a routine of monotony and I feel like people bail when relationships get to this point where reality kicks in and day to day life isn't always fun and happiness and constant excitement. Sometimes you might not always be laughing and dancing and feeling excitement with your partner, sometimes that could last for months at a time. But that's life. It's like people are always looking for the "high" of a new relationship and forget that relationships are work and about dedication and love for your partner through all sorts of times.