You see them smiling at someone and you think that there’s something going on between the two of them and that your relationship is in danger. Even if they tell you, that the respective person is a newly found acquaintance, the heightening jealousy keeps you from believing them. They need to keep reassuring you of your importance to them. But they’d only put up with it for so long before concluding that you don’t trust them. This is what jealousy does to a relationship. It ruins one of the most important things required for a relationship; trust.
Even though jealousy comes naturally, it doesn’t harm you or your relationship as long as you keep it in check.
Reassure your partner
If they’re jealous even when you see no reason for it, reassure them. Try to find out why exactly do they feel so, and then try to fix it. Don’t take it as a sign of lack of trust; instead think of it as their means of expressing the fear of losing you. Give them the attention they need and help them get over the jealousy. Don’t be offended by it, it’d only complicate things even further.