Interpersonal relationships are dynamic systems that change continuously during their existence. Like living organisms, relationships have a beginning, a lifespan, and an end. They tend to grow and improve gradually, as people get to know each other and become closer emotionally, or they gradually deteriorate as people drift apart, move on with their lives and form new relationships with others. One of the most influential models of relationship development was proposed by a psychologist George Levinger. This model was formulated to describe heterosexual, adult romantic relationships, but it has been applied to other kinds of interpersonal relations as well. According to the model, the natural development of a relationship follows five stages:

1. Acquaintance and acquaintanceship – Becoming acquainted depends on previous relationships, physical proximity, first impressions, and a variety of other factors. If two people begin to like each other, continued interactions may lead to the next stage, but acquaintance can continue indefinitely. Another example is the association.
2. Buildup – During this stage, people begin to trust and care about each other. The need for intimacy, compatibility and such filtering agents as common background and goals will influence whether or not interaction continues.
3. Continuation – This stage follows a mutual commitment to quite a strong and close long-term friendship, romantic relationship, or even marriage. It is generally a long, relatively stable period. Nevertheless, continued growth and development will occur during this time. Mutual trust is important for sustaining the relationship.
4. Deterioration – Not all relationships deteriorate, but those that do tend to show signs of trouble. Boredom, resentment, and dissatisfaction may occur, and individuals may communicate less and avoid self-disclosure. Loss of trust and betrayals may take place as the downward spiral continues, eventually ending the relationship. (Alternately, the participants may find some way to resolve the problems and reestablish trust and belief in others.)
5. Ending – The final stage marks the end of the relationship, either by breakups, death or by spatial separation for quite some time and severing all existing ties of either friendship or romantic love.

在TBBT里，Sheldon called Leonard and Raj friends, and Howard a "treasured acquaintance". 最后一集他领诺贝尔奖时致辞：...I was there the moment Leonard and Penny met... [LDOCE][牛津]
1. I made his acquaintance at a party.
2. Most people are nicer than you think on first acquaintance.

scrape (up) an acquaintance with sb: attempt to meet or befriend sb, esp because they may prove useful to one's needs, ambitions, or desires, 套瓷. "Well then, the soultion is simple. I shall befriend him. Kripke! ... Well I think we're off to a terrific start."

quaint:unusual and attractive <= French(clever) <= Latin(known). acquaint: to know. 狡黠(jiǎo xiá)，近/反义词；诡诈，机灵、狡猾，圆滑、正直、憨厚、诚实、古灵精怪