Why can't I resolve the dependencies for GeoTools maven quickstart?

The issue occurs because the org.geotools:gt-shapefile:jar:10-SNAPSHOT does not exist at either http://download.java.net/maven/2 or http://download.osgeo.org/webdav/geotools. All you have to do is define the snapshot repository for the GeoTools as it is mentioned at the GeoTools:Maven Quick Start as the following: -

<repositories>
    <repository>
        <id>maven2-repository.dev.java.net</id>
        <name>Java.net repository</name>
        <url>http://download.java.net/maven/2</url>
    </repository>
    <repository>
        <id>osgeo</id>
        <name>Open Source Geospatial Foundation Repository</name>
        <url>http://download.osgeo.org/webdav/geotools/</url>
    </repository>
    <repository> <!--Add the snapshot repository here-->
        <snapshots>
            <enabled>true</enabled>
        </snapshots>
        <id>opengeo</id>
        <name>OpenGeo Maven Repository</name>
        <url>http://repo.opengeo.org</url>
    </repository>
</repositories>

I hope this may help.

Some of the repositories in above answers have been moved to alternative addresses. Adding the following repositories solved the issue for me.

 <repository>
      <id>osgeo-alt</id>
      <url>https://repo.osgeo.org/repository/release/</url>
 </repository>
 <repository>
       <id>geomajas</id>
       <name>Geomajas Maven Repository</name>
       <url>http://maven.geomajas.org/(http://maven.geomajas.org/)</url>
 </repository>

http://download.java.net/maven/2 repository is gone

New updates, New way to add geotools dependency in your project, copy below xml file in your build.xml

<properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <geotools.version>19-SNAPSHOT</geotools.version>
    </properties>

<dependencies>

        <dependency>
            <groupId>com.opencsv</groupId>
            <artifactId>opencsv</artifactId>
            <version>3.3</version>
        </dependency>


        <dependency>
            <groupId>org.geotools</groupId>
            <artifactId>gt-shapefile</artifactId>
            <version>${geotools.version}</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.geotools</groupId>
            <artifactId>gt-swing</artifactId>
            <version>${geotools.version}</version>
        </dependency>

    </dependencies>

<repositories>

        <repository>
            <id>osgeo</id>
            <name>Open Source Geospatial Foundation Repository</name>
            <url>http://download.osgeo.org/webdav/geotools/</url>
        </repository>
        <repository>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
            <id>boundless</id>
            <name>Boundless Maven Repository</name>
            <url>http://repo.boundlessgeo.com/main</url>
        </repository>
    </repositories>

It works