Reasons:

* Electronegativity: Phosphorus has a higher electronegativity (2.19) than sulfur (2.58). This means that phosphorus has a greater tendency to attract electrons, making it more likely to participate in chemical reactions.

* Ionization Energy: Phosphorus has a lower ionization energy than sulfur. This means that it requires less energy to remove an electron from a phosphorus atom, making it easier to form positive ions and participate in reactions.

* Atomic Size: Phosphorus is smaller than sulfur. This smaller size allows for stronger interactions with other atoms, leading to increased reactivity.

* Availability of d-orbitals: Phosphorus has vacant d-orbitals, which allow for expansion of its valence shell. This allows phosphorus to form more bonds and participate in a wider range of reactions.

Examples:

* Phosphorus readily reacts with oxygen to form oxides, while sulfur reacts more slowly.

* Phosphorus is highly reactive with halogens, while sulfur reacts more slowly.

* Phosphorus forms more stable compounds with metals than sulfur.

Conclusion:

Due to its higher electronegativity, lower ionization energy, smaller atomic size, and availability of d-orbitals, phosphorus is more reactive than sulfur.