There is a saying which goes:
"The toughest thing in the world is not a diamond, but a great vow ".

How can a vow be harder to break than a diamond?

In Buddhism, great vows hold more weight than anything else.

There are many examples of great vows in Buddhism:
the Ten Vows by Samantabhadra Bodhisattva;
the Twelve Vows by the Medicine Buddha;
the Forty-eight Vows by Amitabha;
and Ksitigarbha Bodhisattva's vow that he wouldn 't become a Buddha until hell was empty.

All the Buddhas and Bodhisattvas achieved enlightenment exclusively by making vows.

It is therefore evident that,
if one wants to practice Buddhism and become a Buddha one day,
one must commit to great vows.