1. Electronegativity: Chlorine is highly electronegative, meaning it has a strong attraction for electrons. This makes it eager to gain an electron to achieve a stable octet configuration.

2. Small Atomic Radius: Chlorine has a relatively small atomic radius, which means its nucleus can exert a stronger pull on electrons. This makes it easier for chlorine to attract electrons and form bonds.

3. High Electron Affinity: Chlorine has a high electron affinity, meaning it releases a significant amount of energy when it gains an electron. This release of energy makes the reaction more favorable.

4. Reactivity of Non-metals: As a non-metal, chlorine tends to react with other non-metals and metals to form compounds. This is because it seeks to gain electrons to achieve a stable electron configuration.

5. Existence as a Diatomic Molecule: Chlorine exists as a diatomic molecule (Cl2), which means two chlorine atoms are bonded together. This bond is relatively weak, allowing the molecule to readily break apart and react with other substances.

6. Oxidizing Properties: Chlorine is a strong oxidizing agent. It readily gains electrons from other substances, oxidizing them and causing itself to be reduced.

In summary, chlorine's high electronegativity, small size, high electron affinity, non-metallic nature, diatomic structure, and oxidizing properties all contribute to its high reactivity.

It's important to note that while chlorine reacts readily, it's also a very useful element. It's used in various applications, including water purification, bleach production, and the production of plastics and pharmaceuticals. However, its high reactivity also makes it potentially hazardous, and it should be handled with care.