Just because you say you will do something does not mean that you are also willing to do that thing. Ensuring that both things are true is especially important in the matter of either making or receiving a proposal of marriage.
Think of yourself as a compass with multiple needles:
One needle represents your inner voice (intuition).
One needle represents your thoughts.
One needle represents your actions.
Imagine that any direction on the compass can be “right” or “wrong”, “good” or “bad”, depending upon the situation you are in.
When the needles are not aligned, you will be feckless, confused, and of equivocal goodness.
When all needles are aligned and are pointing in a certain direction, you will be effective.
When all needles are aligned and are pointing in the wrong direction, you will be evil.
When all needles are aligned and are pointing in the right direction, you will be righteous.
In other words, in any situation, each of us should aspire to achieve a state in which our inner voice, our thoughts, and our actions are all working towards the same, good end result.
How do you know what the right direction is in any situation? Find a moral and learned teacher you trust and learn from them.
Allowing yourself to do something (the “will” in the aphorism) can be disastrous if the action conflicts with what you know or feel to be wrong. You cannot be willing to do something unless your thoughts and intuition also support the action you are taking. This especially holds true when confronted with having to make a profound life decision (e.g., whether to give or accept a proposal of marriage). The aphorism suggests how to approach making such a decision.