Sagittarius is a mutable (meaning "changeable") sign, and people born under the mutable signs - Gemini, Virgo and Pisces are the others - are very changeable and easily moved to new thought and activity. Like smoke or steam, their energy escapes constantly and can be dissipated or spread widely. They need variety and are not easy to pin down, thus they do not take on responsibility easily. It's not a lack of initiative or ambition that causes the problem. They realize very quickly that for them responsibility inevitably results in less personal freedom and fewer options...