Lone pair orbitals adjacent electron rich interactions

Carbenes that have singlet ground states (such as CC12) all have electron-rich substituents carrying lone pairs adjacent to the carbene centre. These lone pairs can interact with the p orbital of the carbene to produce a new, lower-energy orbital which the two electrons occupy. This stabilization of the lone pair provides the incentive that the electron in the p orbital needs to pair up in the sp2 orbital. 

---