This must be the same for partners, family and friends who are driving with you; the scream out of their own car windows, “we are trying to get somewhere Parkinson’s, get the f**k out of our way!!”
This is especially true when you are trying to start a relationship (in my case) or maintain a relationship. Your partner (who has potentially known you for years) or potential partner (who has expectations of what they want in a partner) will expect you to drive at a certain speed and with a certain level of skill. As they are wanting to race ahead with you, you are stuck behind the Sunday driver that is Parkinson’s. Eventually you drift apart (or never drift together) and lose sight of each other.
I think we respond to love and being wanted both emotionally and physically; we can normally reciprocate love in the same way. Because Parkinson's undermines us physically and emotionally however and at the same time increases the need to be loved (especially somebody to love our crumbling physical body) it reduces this response and reciprocation to dreadful isolation; the disease stops the normal cycle of being in love: you engage emotionally with someone, they reciprocate, then you engage physically which lays new ground for a deeper emotional connection (in an ideal world). Parkinson's dissolves this possibility both for you and your partner.