Some people are just more receptive than others.
Activity level could also be key. What are you doing while trying to see/hear a spirit?
What are you consuming in the way of food and/or beverages? I've heard that too much caffeine could detract from sensing a spirit, although I'm a caffeine loaded diet soda addict, and that hasn't stopped me from seeing/hearing/sensing them.
Meditation is a good way to relax yourself and open up your mind rather than trying to "force" the issue. Being anxious and/or excited can sometimes make it harder to sense them.
Hope this helps.