Oldest children can be quite perplexing, even psychologically, if not psychically daunting/overwhelming to others. Oldest children are quite uncompromising, even authoritarian in their dealings w/others. Oldest children MUST be number 1 & in the dominant position. Oldest children aren't comfortable when they are in a vulnerable or submissive position.
When one enters into a relationship w/the oldest child in the family, h/she must be aware of the psychology of the oldest child. The oldest child will establish his/her preeminence in the relationship very early. The person entering the relationship will know where h/she stands regarding the oldest child. Oldest children refuse to suffer fools gladly so the person in such a relationship had better be up to par.
However, there are oldest children who are attracted to those who are somewhat "lesser". It isn't uncommon for oldest children to have relationships, even friendships w/ those who have lesser education, lower job status, & other unequal statuses. Oldest children love to be the boss, the dominant one. They feel quite comfortable w/ those who are considered to be "lesser". They can be quite unnerved with those who are equal &/or superior to them.
Oldest children feel that they are right & will go to any lengths to convince others of this. They will assert that their idea &/or ideologies are the only legitimate ones while others are in errors because they present different perspectives on such. They aren't above using force or other forms of emotional & psychological manipulations to convince others that THEY are right.
Oldest children because of their familial position are doing for others. They do this because they want to be thought as the good person in the relationship. Oftentimes, they are doormats to people who take advantage of them because oldest children have the habit of putting themselves LAST while they put others FIRST. They are loathe to be assertive in expressing their needs. This results to them adopting a passive-aggressive mentality, taking out their rage on those in their immediate circle instead of confronting the party/parties who are taking advantage of them.