The following is the proposition 3.3 of folland "A Course in Abstract Harmonic Analysis" book. please Explain its proof in more details:
I do not know the cause of contradiction. that is, how maximality is violated?

The following is the proposition 3.3 of folland "A Course in Abstract Harmonic Analysis" book. please Explain its proof in more details:
I do not know the cause of contradiction. that is, how maximality is violated?

Suppose $u$ is orthogonal to all $M_a$s. Then the cyclic rep $U$ generated by $u$ is orthogonal to the $M_a$s.
Therefore the collection $\{M_a\}_{a\in\cal A}\cup\{U\}$ is also a collection of mutually orthogonal cyclic reps, and it is strictly greater than the collection $\{M_a\}_{a\in\cal A}$. (The partial order here is inclusion.) But our hypothesis was that $\{M_a\}_{a\in\cal A}$ was the greatest possible collection of mutually orthogonal cyclic reps. How can it be the greatest possible when there is one greater? It can't be - that's a contradiction!