There is a lot encompassed in this simple thought. At first, it might seem like a throw-away line. "Be yourself." We hear this kind of drivel all the time in our daily lives. Everyone wants us to just be ourselves. It's all over billboards and other advertising. Musicians beseech it in a lot of empowerment songs. Finally, we just say "FINE! I'm being myself! Geez! Get off my back!"
It's the second line that brings the first line into more focus here, though. "Especially, do not feign affection."
To me, this suggests something deeper - something that speaks of Truth. Being ourselves is a lot more than picking a series of attributes and hobbies, a sense of fashion or style, slapping a name on it, and calling it "myself." When we are truly being ourselves, we are admitting who we are. We are admitting all of our flaws and virtues, the things we love and hate, perhaps even despite ourselves.
Being yourself does not necessarily mean you cannot work on yourself. But, to work on yourself, you also need to admit to yourself who you are.
And, one of the top priorities in being who you are is emphasized in Ehrmann's charge to us - "do not feign affection." In being yourself, and admitting who you are, you are casting off deception. This means being clear about your affections and your intentions. Feigning affection, feigning friendship - this causes so much unnecessary strife in our lives. To reduce it to modern terms, it means drama, and gossip, and discord. How much strife is really caused in your life by you or people you know spending time with people they dislike while pretending to like them?
On a deeper level, it can also cause real devastation. Feigning affection - feigning love - can ultimately lead to ruin. Even if the deception is caused by good intentions - not wanting to hurt someone's feelings, not wanting to cause them pain - it is much, much worse when a relationship has continued down a path that it shouldn't have because one or both people simply were not being honest with themselves or each other about love. When things ultimately come to a head - and they will - it's that much worse for having gone on too long. This does not mean you cannot work on a relationship where love is in jeopardy. On the contrary - admitting there is a problem and then knowing you can work on it can ultimately save it. But it also means honesty.
This does not mandate a lack of civility. Ehrmann advocates politeness and civility earlier in Desiderata - "As far as possible without surrender be on good terms with all persons." But it does mean honesty and communication - being open rather than deceptive, addressing problems to people directly rather than talking about them. And being honest with yourself, your family, your significant other, your friends, about love.