The human symbolises the search for that which lies beyond animal instinct. The two are always together - meaning, philosophy, religion, are divorced from instinct at their peril. Then you get the philosophers and preachers who exhibit great powers of mind but cannot touch the heart. Sagittarius is a fire sign, and like the other fire signs, Aries and Leo, needs freedom of movement. This need is particularly highlighted in Sagittarius by its emergence from the intensity of Scorpio.
Travel is one traditional Sagittarian way of broadening one s horizons.
If you are close to someone whose chart is strong in Sagittarian energy and wish him or her to remain faithful, then you are more likely to succeed if you recognise this need to escape at regular intervals. Significant planets, e.g. the Moon or Venus, in Capricorn in your chart would indicate that you are frequently drawn towards situations which you may find limiting in a way you experience as alien. Relationships may be scenarios in which you feel restricted from living life to the full, with the freedom you may feel is appropriate to you. But such features in your chart would show that some part of your nature longs for containment of some kind, even the freedom to live life within safe boundaries for a time. The above is just a brief sketch of the sort of qualities Sagittarius should be aiming for.