Question

bond angle in NH3 is more than in H2O

Answer 1

Water H2O has bond angle = 104.5 deg.
NH3 ammonia has bond angle =107.8 deg.

   Bond angle in general is inversely proportional to each of L, ENC and ENS.
          L is the number of lone pairs,
          ENC is the electronegativity of the central atom.
          ENS is the electronegativity of surrounding atom.
          Double bonds and triple bonds do not affect bond angles.  Only single bonds do.

   ENS (Hydrogen) is same in both.  L = 1 in NH3.  And L = 2 in H2O.   ENC is more for Oxygen.  These factors result in the smaller bond angle for water molecules than in Ammonia molecule.

   In a Lone pair: lp, the electrons are closer to central atom than in a bond pair: bp.  Lp repels the neighbouring bonds more.
         Lp-lp repulsion >  lp-bp repulsion > bp-bp repulsion.

    More the number of lone pairs in the central atom, more they repel the neighboring atoms.  So more they cause the neighbor atoms to come closer.  So the bonds come together. So the bond angle reduces.

     Another reason we could give is that there is a greater s-character in the N-H bonds (hybrid orbitals) in ammonia.  In water molecules the O-H bonds (hybridized orbitals) have less of s-character.

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More examples for clarity of application of above principles.

1.    SO2 has resonant structure with one double bond and one single bond.  So number of lone pairs is effectively less than  1.  So its bond angle is high 119 deg.
2.   Methane CH4 has no lone pairs.  It is tetrahedral.  So it has a higher bond angle of 109.5 deg.
3.  On the other hand,  NF3 and PF3 have a different justification.   Fluorine has a high  electro-negativity.  So an electron pair in one F attracts an electron pair in another F.  So the bonds come together as Fluorine atoms come together.   So bond angle is less than in Ammonia.

4.    Between PF3 and PH3, the PF3 develops a partial double bond due to resonance structures.  So the effective number of lone pairs is less.   So the bond angle in PF3 is more than in PH3.

5.   Between  H2S  and  H2O, Oxygen is more electro-negative.  But the bond angle in Water (104.5 deg) is more than in H2S (92.3 deg).   Due to formation of hybrid orbitals, the bonds should be at 90 deg.  But Sulphur being bigger, it has more electron pairs.   There is more repulsion against the Hydrogen atoms.  Hydrogen atoms in H2S have less positive charge on them than in water.  So the Hydrogen atoms do not repel each other as much.  So the bonds are closer with less bond angle.