Why do bond angles change?

Hint: The angle between the lines representing the directions of the bonds that are the orbitals containing the bonding electrons is known as bond angle. It is expressed in degrees, minutes and seconds.

Complete answer:
As we have already seen, what is the Bond angle? Now, we need to understand what are the factors that affect the bond angle and cause change in the angle.
Hybridisation: Hybridisation or we can say $\% $ s character in the hybrid orbital affects the bond angle. More the s-character, more will be the bond angle. We can say that:
Bond Angle $\alpha $ $\% $s character
This can be explained as the s orbital is closer to the nucleus, the electrons in the s orbital will be tightly bound. These electrons will then repel each other, this repulsion will result in an increase of bond angle.
Bond angle depends on the state of hybridisation of the central atom.
Let’s see an example: $C{H_4}$ is $s{p^3}$hybridised. The bond angle observed is ${109^0}$
$BeC{l_2}$ is $sp$ hybridised. The bond angle observed is ${180^0}$
Lone pair repulsion: The presence of the lone pair on the central atom affects the bond angle. Lone pairs on the central atom tend to repel the bonded pair of electrons. Thus, decreasing the bond angle. For example: $C{H_4}$ has no lone pairs. The bond angle observed is ${109^0}$
${H_2}O$ has 2 lone pairs. The bond angle observed is ${104.8^0}$.
Electronegativity of central atom: More electronegative the central atom is more will be the bond angle. Electronegativity is directly proportional to the bond angle.
For example: The bond angle of $N{H_3}$ is greater than that of $P{H_3}$. This is because Nitrogen is more electronegative than phosphorus.

Note:
The electronegativity of the substituent attached to the central atom also affects the bond angle. More electronegative substituent, decreases the electron density on the central atom. Thus, decreasing the bond angle.