''....they shall make for themselves fringes on the corners of their garments, throughout their generations, and they shall affix a thread of sky blue [wool] on the fringe of each corner.'' [Shelach 15:38]
The turquoise strands of the tzitzit allude to ''abandoning evil'' [negative] and serving G-d out of fear; the white strands allude to ''doing good'' [positive] and serving G-d out of love.
In the current era, the turquoise dye [techelet] is not available to us, leaving all eight strands white. This teaches us that nowadays our emphasis in serving G-d should be on love and positivity.
Source: Likutei Sichos vol 8 Lubavitcher Rebbe