In a certain degree, I believe Freud only wants to argue that "the sense of security" is the only final goal or sense that people are pursuing for. With this sense, I found it is easier for me to understand Freudian theory.
I still agree with Lily's suggestion about revenge; however, I would take revenge for getting a sense of security, after infants leave their mothers. Taking Lily's example, I think throwing toys can be interpreted into that the infant wants more attention from mothers, because it no longer can stay with mother all day long at every moment as it did in womb. Actually this idea is pretty much close to Lacanian theory, but I do not completely agree that the performance of showing an infant's needs can only be acted out in a verbal form. Rather, acting is also one way for infants to require for mother's further attention such as waving arms, turning head, and stamping legs, when they are still strained on beds because their biological bodies have not developed well yet.
As for pain, I am wondering if infants know what it is. Through my two nieces and one nephew, I found that sometimes they do not know what pain was until I asked them "is it pain?" Here are possible reasons: 1. They probably do not know that feeling called pain. 2. They probably do not feel the pain. This means that they have no never to feel the pain either psychologically or biologically. 3. They do not regard it pain in the condition that they know how pain feels like. With these assumptions, I am wondering about infants' purpose for crying. When infants cry after fall or with some injures, for example, do they cry for "the feeling of pain" or they actually cry for "something unknown happen to me, I feel scared and insecure." According to Freud, I think the latter might be the answer.