Why do transition elements form complexes?

Hint: Transition metal ions form coordination complex because they have empty valence-shell orbitals that can accept pairs of electrons from a Lewis base. It forms complexes due to the presence of vacant d orbitals.

Complete answer:
Transition metals are d-block elements with valence shell configuration $n{s^2}(n - 1){d^{1 - 10}}$
These metals and their ions easily coordinate with a number of negative ions or neutral molecules having lone pairs of electrons to form a complex.
Their tendency to form complexes is due to two reasons.
1. Small size and high nuclear charge of these metals.
2. Availability of vacant d-orbitals of suitable energy to accommodate lone pairs of electrons donated by the ligands.
Transition elements have a tendency to form complexes more than s and p block elements.
So they are able to form complexes with the groups which are able to donate an electron pair.
The cations of d-block elements have a strong tendency to form complexes.
Hence transition element form complexes.

Note:
Students can get confused at vacant d-orbital, make sure that transition elements are the more reactive due presence of vacant d-orbital. The presence of 5 d orbitals, which can accommodate a total of 10 electrons, in each transition metal atom helps it accept the electrons donated by the electron-rich ligands since the donated electrons can be accommodated in the vacant d orbitals of the transition metal atom.