the lone pairs plus the two electrons(the -ve charge) make the ring highly activated.

when O donates its l.p it acquires +ve charge.. if H is attached 2 O then the resulatant is unstable!!(bcoz Ois more electro-ve than H but gets+ve!!)

in phenoxide ion O has an enormous amt. of -ve charge ther4 it donates its l.p more freely........an hence mor stable.....

moreover, the canonical strucures of phenoxide ion are equivalent(unlike phenol)

AND.................

EQUIVALENT STRUCTURE MEANS MORE STABILITY OF THE HYBRID...........................

If you draw the resonating structure of phenol and phenoxide ion, you will see both have 5 resonating structure. But  you will see that out of 5 resonating structure of phenol 3 contain a positive charge on the oxigen of -OH group, while a negative charge on one of the carbon atom in the ring. while phenoxide ion does not contain any positive charge but the negative charge of oxygen is delocalised on the whole strucrure. since charge separation destabilises amolecule therefore phenol is less stable than phenoxide ion.