Quite possibly the best piece of advice I've ever heard or could ever give in regards to building strong relationships whether it be business, friendships, or romantic. Without a doubt this is a constant battle BUT being vulnerable is the heart of healthy relationships - you need it to exist. It is only viewed as a weakness to those who merely see it for it's literal meaning.
Vulnerability is not what you think it is, vulnerability is much more. Vulnerability is vital.
PS - For more on vulnerability check out Brene Brown's TED Talk. It's one of the most watched TED talks of all time. A true testament to the power of vulnerability.