Jealousy is a natural human emotion that can be caused by various circumstances. People often begin to feel jealous when they perceive a threat to their relationship or possessions. There are various words and phrases that can be used to describe this feeling. One synonym for "begin to be jealous" is "become envious," which implies a sense of longing or desire for something that someone else has. Another synonym is "feel possessive," which denotes a sense of ownership and the need to protect what is rightfully yours. Alternatively, one might "feel territorial," which refers to a primal instinct to assert dominance over a specific area. Whatever the synonym, jealousy can be a challenging emotion to manage, and it's essential to address the underlying causes to move past it.