Although Jesus made a bunch of promises and the big command to love one another, Jesus also told them what could possibly happen. Jesus said these things so the disciples could be prepared to face said scenarios.
Jesus told the disciples that they are "not of the world" even though "[Jesus] chose [them] out of the world" (15:19). In context, being "not of the world" means "not conforming to worldly character (focused on riches and popularity)", which is supported by how one of the meanings of the Greek word for "world" (Strong 2889) that means "worldly affairs". Not to mention that eventually, Jesus would get killed by not only Judean leaders but also the Roman executioners. Thus, while the world at that time (and even today) focused on riches, popularity, and power, the disciples are called to focus on humbling themselves in service.
Jesus told the disciples that they are "not of the world" to let them know of what they would face. In particular, the disciples would face lots of pressure from the world. Jesus pointed out how "the world would love its own" and how the world "hated [Jesus] before it hated [the disciples]" (15:18-19). Thus, when hating Jesus, the world also hates Jesus's disciples. In fact, Jesus proclaimed that since a servant isn't greater than a master, and that those at that time who persecute Jesus would also persecute the disciples (15:20). On the flip side, the world that loves Jesus would keep the word of the disciples. In particular, Jesus might be referencing the account of the disciples regarding Jesus (and four of them wrote books that talk about Jesus).
Today, while in many areas there are freedoms to follow Jesus, there are still instances of people throwing Jesus down. As Christ-followers don't operate by worldly standards of greed and selfishness, we don't have to be surprised whenever we face pressure from the world. In fact, we can know that Jesus loves us a lot.