Why are users with less than 125 reputation not allowed to cast their vote? Don't they have right to express their feelings?
Users with over just 25 reputation are allowed to cast upvotes.
It is only downvoting that requires 125 reputation.
There are good reasons for this, primarily, it's because the SE think's it's more beneficial to focus on upvotes as you are still becoming used to the community. It should take some time for you to recognize how the community functions, what is good content, and what is bad content in the SE format.
The site is community driven, so the members who primarily drive the voting process should at least be users familiar enough to gain 125 rep somehow.
The downvoting privilege page I linked also gives good advice for what to do instead of downvoting:
What are the alternatives to down-voting?
The up-vote privilege comes first because that's what you should focus on: pushing great content to the top. Down-voting should be reserved for extreme cases. It's not meant as a substitute for communication and editing.
Instead of voting down:
If the post is spammy or offensive, flag it.
If the question is duplicate or off-topic, flag it for moderator attention.
If something is wrong, please leave a comment or edit the post to correct it.