Unsolicited advice is seen as criticism in disguise
There is a wide range of reasons why unsolicited advice is perceived as criticism in disguise. It's often because unsolicited advice-givers are feeling compelled to make a recommendation, and they lack awareness of why their suggestions are unwarranted. In some cases, they are just seeking control and order, and they feel no need to explain their motives.
Sometimes we see unsolicited advice as criticism, but it's also often given without expectation. The person offering the advice may be a nagging relative, someone who is wishing to win us over with love or acceptance. Alternatively, a friend might give us advice without any expectation of reciprocation. The point of criticism is to point out mistakes, not to praise or insult.
It weakens communication
Unsolicited advice can weaken communication and create negative consequences. It can also lead to relationship damage. Avoid giving unsolicited advice to your partner. Instead, focus on communicating your feelings directly. Then, choose the right medium for your communication. For example, talking directly to someone is much better than sending them an email.