Doesn't the Bible say not to be jealous? But then in more than one place it says God is a jealous God.
Jealousy means "Vigilant guarding of a possession." It is the desire to hold on to what is yours for your own personal use. Sometimes we wrongfully believe that what someone else has is our own exclusive possession. For example, a husband may believe he alone should be the focus of his wife’s attention. He then is jealous of his wife’s attention and envious of others who hold his wife’s attention.
But sometimes jealousy is used in a good sense. God is a jealous God. "You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; you shall not bow down to them nor serve them. For I, the LORD your God, am a jealous God, visiting the iniquity of the fathers on the children to the third and fourth generations of those who hate Me, but showing mercy to thousands, to those who love Me and keep My commandments" (Exodus 20:4-6). God’s people belong to him exclusively. We are not to share our time or possessions with other gods, such as idols (Matthew 6:24). We are a people set apart for God’s own possession (I Peter 2:9-10).
Similarly, preachers and elders may be jealous to guard the purity of God’s people (II Corinthians 11:2-3).
The love between husband and wife is a jealous love. "Set me as a seal upon your heart, as a seal upon your arm; for love is as strong as death, jealousy as cruel as the grave; its flames are flames of fire, a most vehement flame. Many waters cannot quench love, nor can the floods drown it. If a man would give for love all the wealth of his house, it would be utterly despised" (Song of Solomon 8:6-7). A husband does not want to share the love of his wife with another man, nor does a woman want to share the love of her husband with another woman. The marriage vows makes that relationship exclusive. Like death, the love between a husband and wife does not easily relinquish it’s hold.
The problem with jealousy is that it is a strong passion. In the Hebrew the word means "fire." Like real fire, it has a useful place, but it can easily and quickly get out of control if we are not careful. It is very easy to slip and become jealous about things that really don't belong to you, though we think they should.