A person can like many other persons, but (usually) he
can love only another person (at least over a span of time with variable length).
If you like somebody and you don't enjoy her company, it's ok, but in the case
of the love feeling unhappiness usually arises.
Love is definetly a severe perturbation of our mental state, in certain cases it is a illness, but it's probably the only severe illness that might bring new life on this green and blue piece of stone that is hosting us, instead of the end of a life which is the usual aim of other severe illnesses.