If guys get nervous when you approach them, it is very natural.
Why?
When you speak to someone, you always enter in their personal space.
This happens naturally.
Without realizing it or truly wanting it, you use your "conquering power" when you start chatting with someone.
If you want a guy to feel comfortable, give him space.
Give him time to adapt to your presence.
Humor and teasing is a good way to get everyone relaxed.
Chat a bit, let go.
Play with that.
There is nothing to worry about.
Show them there is nothing you want from them.
People tend to build walls around them.
It's a sphere of protection.
When you break through that protection, they naturally feel defenseless.
There is a new balance of power and energy.
It can take a couple of minutes for this energy to find a new balance.
There is another element as well: they could truly be excited by your presence and talking to you.
Their lack of confidence can show as if they were uncomfortable.
Can you see that?
It is true, the one who takes the initiative ( specially if you show lots of assertiveness) uses this conquering power.
They basically go beyond their comfort zone and connect.
The one who is waiting and receptive is within a sphere of comfort and by coming into their sphere, you challenge this comfort.
Again, there is nothing wrong with that.
Give it a couple of minutes to readapt.
Play with humor, teasing and eventually show less assertiveness in your approach.